diff --git a/script/test.py b/script/test.py
index 8261f45c..0a14e99a 100755
--- a/script/test.py
+++ b/script/test.py
@@ -17,7 +17,7 @@ EXPECT_ERROR_PATTERN = re.compile(r'// expect error')
 EXPECT_ERROR_LINE_PATTERN = re.compile(r'// expect error line (\d+)')
 EXPECT_RUNTIME_ERROR_PATTERN = re.compile(r'// expect runtime error: (.+)')
 ERROR_PATTERN = re.compile(r'\[.* line (\d+)\] Error')
-STACK_TRACE_PATTERN = re.compile(r'\[.* line (\d+)\] in \(script\)')
+STACK_TRACE_PATTERN = re.compile(r'\[.* line (\d+)\] in')
 SKIP_PATTERN = re.compile(r'// skip: (.*)')
 NONTEST_PATTERN = re.compile(r'// nontest')
 
diff --git a/src/wren_core.c b/src/wren_core.c
index c9472135..a1215ecb 100644
--- a/src/wren_core.c
+++ b/src/wren_core.c
@@ -224,18 +224,13 @@ DEF_NATIVE(fiber_new)
   RETURN_OBJ(newFiber);
 }
 
-DEF_NATIVE(fiber_isDone)
-{
-  ObjFiber* runFiber = AS_FIBER(args[0]);
-  RETURN_BOOL(runFiber->numFrames == 0);
-}
-
-DEF_NATIVE(fiber_run)
+DEF_NATIVE(fiber_call)
 {
   ObjFiber* runFiber = AS_FIBER(args[0]);
 
-  if (runFiber->numFrames == 0) RETURN_ERROR("Cannot run a finished fiber.");
-
+  if (runFiber->numFrames == 0) RETURN_ERROR("Cannot call a finished fiber.");
+  if (runFiber->caller != NULL) RETURN_ERROR("Fiber has already been called.");
+  
   // Remember who ran it.
   runFiber->caller = fiber;
 
@@ -248,11 +243,12 @@ DEF_NATIVE(fiber_run)
   return PRIM_RUN_FIBER;
 }
 
-DEF_NATIVE(fiber_run1)
+DEF_NATIVE(fiber_call1)
 {
   ObjFiber* runFiber = AS_FIBER(args[0]);
 
-  if (runFiber->numFrames == 0) RETURN_ERROR("Cannot run a finished fiber.");
+  if (runFiber->numFrames == 0) RETURN_ERROR("Cannot call a finished fiber.");
+  if (runFiber->caller != NULL) RETURN_ERROR("Fiber has already been called.");
 
   // Remember who ran it.
   runFiber->caller = fiber;
@@ -273,15 +269,69 @@ DEF_NATIVE(fiber_run1)
   return PRIM_RUN_FIBER;
 }
 
+DEF_NATIVE(fiber_isDone)
+{
+  ObjFiber* runFiber = AS_FIBER(args[0]);
+  RETURN_BOOL(runFiber->numFrames == 0);
+}
+
+DEF_NATIVE(fiber_run)
+{
+  ObjFiber* runFiber = AS_FIBER(args[0]);
+
+  if (runFiber->numFrames == 0) RETURN_ERROR("Cannot run a finished fiber.");
+
+  // If the fiber was yielded, make the yield call return null.
+  if (runFiber->caller == NULL && runFiber->stackSize > 0)
+  {
+    runFiber->stack[runFiber->stackSize - 1] = NULL_VAL;
+  }
+
+  // Unlike run, this does not remember the calling fiber. Instead, it
+  // remember's *that* fiber's caller. You can think of it like tail call
+  // elimination. The switched-from fiber is discarded and when the switched
+  // to fiber completes or yields, control passes to the switched-from fiber's
+  // caller.
+  runFiber->caller = fiber->caller;
+
+  return PRIM_RUN_FIBER;
+}
+
+DEF_NATIVE(fiber_run1)
+{
+  ObjFiber* runFiber = AS_FIBER(args[0]);
+
+  if (runFiber->numFrames == 0) RETURN_ERROR("Cannot run a finished fiber.");
+
+  // If the fiber was yielded, make the yield call return the value passed to
+  // run.
+  if (runFiber->caller == NULL && runFiber->stackSize > 0)
+  {
+    runFiber->stack[runFiber->stackSize - 1] = args[1];
+  }
+
+  // Unlike run, this does not remember the calling fiber. Instead, it
+  // remember's *that* fiber's caller. You can think of it like tail call
+  // elimination. The switched-from fiber is discarded and when the switched
+  // to fiber completes or yields, control passes to the switched-from fiber's
+  // caller.
+  runFiber->caller = fiber->caller;
+
+  return PRIM_RUN_FIBER;
+}
+
 DEF_NATIVE(fiber_yield)
 {
   if (fiber->caller == NULL) RETURN_ERROR("No fiber to yield to.");
 
+  ObjFiber* caller = fiber->caller;
+  fiber->caller = NULL;
+
   // Make the caller's run method return null.
-  fiber->caller->stack[fiber->caller->stackSize - 1] = NULL_VAL;
+  caller->stack[caller->stackSize - 1] = NULL_VAL;
 
   // Return the fiber to resume.
-  args[0] = OBJ_VAL(fiber->caller);
+  args[0] = OBJ_VAL(caller);
   return PRIM_RUN_FIBER;
 }
 
@@ -289,8 +339,11 @@ DEF_NATIVE(fiber_yield1)
 {
   if (fiber->caller == NULL) RETURN_ERROR("No fiber to yield to.");
 
+  ObjFiber* caller = fiber->caller;
+  fiber->caller = NULL;
+
   // Make the caller's run method return the argument passed to yield.
-  fiber->caller->stack[fiber->caller->stackSize - 1] = args[1];
+  caller->stack[caller->stackSize - 1] = args[1];
 
   // When the yielding fiber resumes, we'll store the result of the yield call
   // in its stack. Since Fiber.yield(value) has two arguments (the Fiber class
@@ -299,7 +352,7 @@ DEF_NATIVE(fiber_yield1)
   fiber->stackSize--;
 
   // Return the fiber to resume.
-  args[0] = OBJ_VAL(fiber->caller);
+  args[0] = OBJ_VAL(caller);
   return PRIM_RUN_FIBER;
 }
 
@@ -966,16 +1019,17 @@ void wrenInitializeCore(WrenVM* vm)
   NATIVE(vm->boolClass, "toString", bool_toString);
   NATIVE(vm->boolClass, "!", bool_not);
 
+  // TODO: Make fibers inherit Sequence and be iterable.
   vm->fiberClass = defineClass(vm, "Fiber");
   NATIVE(vm->fiberClass->metaclass, " instantiate", fiber_instantiate);
   NATIVE(vm->fiberClass->metaclass, "new ", fiber_new);
   NATIVE(vm->fiberClass->metaclass, "yield", fiber_yield);
   NATIVE(vm->fiberClass->metaclass, "yield ", fiber_yield1);
+  NATIVE(vm->fiberClass, "call", fiber_call);
+  NATIVE(vm->fiberClass, "call ", fiber_call1);
   NATIVE(vm->fiberClass, "isDone", fiber_isDone);
   NATIVE(vm->fiberClass, "run", fiber_run);
   NATIVE(vm->fiberClass, "run ", fiber_run1);
-  // TODO: Primitives for switching to a fiber without setting the caller.
-  // (I.e. symmetric coroutines.)
 
   vm->fnClass = defineClass(vm, "Fn");
 
diff --git a/test/fiber/call.wren b/test/fiber/call.wren
new file mode 100644
index 00000000..4acd00b4
--- /dev/null
+++ b/test/fiber/call.wren
@@ -0,0 +1,7 @@
+var fiber = new Fiber {
+  IO.print("fiber")
+}
+
+IO.print("before") // expect: before
+fiber.call         // expect: fiber
+IO.print("after")  // expect: after
diff --git a/test/fiber/call_direct_reenter.wren b/test/fiber/call_direct_reenter.wren
new file mode 100644
index 00000000..a4d27515
--- /dev/null
+++ b/test/fiber/call_direct_reenter.wren
@@ -0,0 +1,7 @@
+var fiber
+
+fiber = new Fiber {
+  fiber.call // expect runtime error: Fiber has already been called.
+}
+
+fiber.call
diff --git a/test/fiber/call_indirect_reenter.wren b/test/fiber/call_indirect_reenter.wren
new file mode 100644
index 00000000..da889522
--- /dev/null
+++ b/test/fiber/call_indirect_reenter.wren
@@ -0,0 +1,12 @@
+var a
+var b
+
+a = new Fiber {
+  b.call // expect runtime error: Fiber has already been called.
+}
+
+b = new Fiber {
+  a.call
+}
+
+b.call
diff --git a/test/fiber/call_return_implicit_null.wren b/test/fiber/call_return_implicit_null.wren
new file mode 100644
index 00000000..bdf9ac34
--- /dev/null
+++ b/test/fiber/call_return_implicit_null.wren
@@ -0,0 +1,6 @@
+var fiber = new Fiber {
+  IO.print("fiber")
+}
+
+var result = fiber.call // expect: fiber
+IO.print(result)        // expect: null
diff --git a/test/fiber/call_return_value.wren b/test/fiber/call_return_value.wren
new file mode 100644
index 00000000..fa871c97
--- /dev/null
+++ b/test/fiber/call_return_value.wren
@@ -0,0 +1,7 @@
+var fiber = new Fiber {
+  IO.print("fiber")
+  return "result"
+}
+
+var result = fiber.call // expect: fiber
+IO.print(result)        // expect: result
diff --git a/test/fiber/call_when_done.wren b/test/fiber/call_when_done.wren
new file mode 100644
index 00000000..33f1705d
--- /dev/null
+++ b/test/fiber/call_when_done.wren
@@ -0,0 +1,6 @@
+var fiber = new Fiber {
+  IO.print("call")
+}
+
+fiber.call // expect: call
+fiber.call // expect runtime error: Cannot call a finished fiber.
diff --git a/test/fiber/call_with_value.wren b/test/fiber/call_with_value.wren
new file mode 100644
index 00000000..16d78fe0
--- /dev/null
+++ b/test/fiber/call_with_value.wren
@@ -0,0 +1,9 @@
+var fiber = new Fiber {
+  IO.print("fiber")
+}
+
+// The first value passed to the fiber is ignored, since there's no yield call
+// to return it.
+IO.print("before")    // expect: before
+fiber.call("ignored") // expect: fiber
+IO.print("after")     // expect: after
diff --git a/test/fiber/call_with_value_direct_reenter.wren b/test/fiber/call_with_value_direct_reenter.wren
new file mode 100644
index 00000000..05804b08
--- /dev/null
+++ b/test/fiber/call_with_value_direct_reenter.wren
@@ -0,0 +1,7 @@
+var fiber
+
+fiber = new Fiber {
+  fiber.call(2) // expect runtime error: Fiber has already been called.
+}
+
+fiber.call(1)
diff --git a/test/fiber/call_with_value_indirect_reenter.wren b/test/fiber/call_with_value_indirect_reenter.wren
new file mode 100644
index 00000000..d31c5a61
--- /dev/null
+++ b/test/fiber/call_with_value_indirect_reenter.wren
@@ -0,0 +1,12 @@
+var a
+var b
+
+a = new Fiber {
+  b.call(3) // expect runtime error: Fiber has already been called.
+}
+
+b = new Fiber {
+  a.call(2)
+}
+
+b.call(1)
diff --git a/test/fiber/call_with_value_when_done.wren b/test/fiber/call_with_value_when_done.wren
new file mode 100644
index 00000000..c3fc2f7d
--- /dev/null
+++ b/test/fiber/call_with_value_when_done.wren
@@ -0,0 +1,6 @@
+var fiber = new Fiber {
+  IO.print("call")
+}
+
+fiber.call(1) // expect: call
+fiber.call(2) // expect runtime error: Cannot call a finished fiber.
diff --git a/test/fiber/closure.wren b/test/fiber/closure.wren
index 1d949b92..3dcfea07 100644
--- a/test/fiber/closure.wren
+++ b/test/fiber/closure.wren
@@ -17,9 +17,9 @@ var closure
   }
 }
 
-fiber.run    // expect: before
+fiber.call   // expect: before
 closure.call // expect: before
-fiber.run
+fiber.call
 closure.call // expect: after
-fiber.run    // expect: after
+fiber.call   // expect: after
 closure.call // expect: final
diff --git a/test/fiber/is_done.wren b/test/fiber/is_done.wren
index 945f0c28..31489b90 100644
--- a/test/fiber/is_done.wren
+++ b/test/fiber/is_done.wren
@@ -5,7 +5,7 @@ var fiber = new Fiber {
 }
 
 IO.print(fiber.isDone)  // expect: false
-fiber.run               // expect: 1
+fiber.call              // expect: 1
 IO.print(fiber.isDone)  // expect: false
-fiber.run               // expect: 2
+fiber.call              // expect: 2
 IO.print(fiber.isDone)  // expect: true
diff --git a/test/fiber/resume_caller.wren b/test/fiber/resume_caller.wren
index 14765943..1bdd88fc 100644
--- a/test/fiber/resume_caller.wren
+++ b/test/fiber/resume_caller.wren
@@ -4,12 +4,12 @@ var b = new Fiber {
 
 var a = new Fiber {
   IO.print("begin fiber a")
-  b.run
+  b.call
   IO.print("end fiber a")
 }
 
 IO.print("begin main")
-a.run
+a.call
 IO.print("end main")
 
 // expect: begin main
diff --git a/test/fiber/run.wren b/test/fiber/run.wren
index 9c9baca6..c37ace17 100644
--- a/test/fiber/run.wren
+++ b/test/fiber/run.wren
@@ -4,6 +4,6 @@ var fiber = new Fiber {
 
 IO.print("before") // expect: before
 fiber.run          // expect: fiber
-IO.print("after")  // expect: after
 
-// TODO: Test handles error if fiber tries to run itself.
+// This does not get run since we exit when the run fiber completes.
+IO.print("nope")
diff --git a/test/fiber/run_direct_reenter.wren b/test/fiber/run_direct_reenter.wren
new file mode 100644
index 00000000..a56a7555
--- /dev/null
+++ b/test/fiber/run_direct_reenter.wren
@@ -0,0 +1,10 @@
+var fiber
+
+fiber = new Fiber {
+  IO.print(1) // expect: 1
+  fiber.run
+  IO.print(2) // expect: 2
+}
+
+fiber.call
+IO.print(3) // expect: 3
\ No newline at end of file
diff --git a/test/fiber/run_indirect_reenter.wren b/test/fiber/run_indirect_reenter.wren
new file mode 100644
index 00000000..5bfc185c
--- /dev/null
+++ b/test/fiber/run_indirect_reenter.wren
@@ -0,0 +1,20 @@
+var a
+var b
+
+a = new Fiber {
+  IO.print(2)
+  b.run
+  IO.print("nope")
+}
+
+b = new Fiber {
+  IO.print(1)
+  a.run
+  IO.print(3)
+}
+
+b.call
+// expect: 1
+// expect: 2
+// expect: 3
+IO.print(4) // expect: 4
diff --git a/test/fiber/run_return_implicit_null.wren b/test/fiber/run_return_implicit_null.wren
deleted file mode 100644
index 4e406b1a..00000000
--- a/test/fiber/run_return_implicit_null.wren
+++ /dev/null
@@ -1,6 +0,0 @@
-var fiber = new Fiber {
-  IO.print("fiber")
-}
-
-var result = fiber.run // expect: fiber
-IO.print(result)       // expect: null
diff --git a/test/fiber/run_return_value.wren b/test/fiber/run_return_value.wren
deleted file mode 100644
index c8adc517..00000000
--- a/test/fiber/run_return_value.wren
+++ /dev/null
@@ -1,7 +0,0 @@
-var fiber = new Fiber {
-  IO.print("fiber")
-  return "result"
-}
-
-var result = fiber.run // expect: fiber
-IO.print(result)       // expect: result
diff --git a/test/fiber/run_when_done.wren b/test/fiber/run_when_done.wren
index 24231fb1..7d680e3c 100644
--- a/test/fiber/run_when_done.wren
+++ b/test/fiber/run_when_done.wren
@@ -1,6 +1,13 @@
-var fiber = new Fiber {
+var a = new Fiber {
   IO.print("run")
 }
 
-fiber.run // expect: run
-fiber.run // expect runtime error: Cannot run a finished fiber.
+// Run a through an intermediate fiber since it will get discarded and we need
+// to return to the main one after a completes.
+var b = new Fiber {
+  a.run
+  IO.print("nope")
+}
+
+b.call // expect: run
+a.run // expect runtime error: Cannot run a finished fiber.
diff --git a/test/fiber/run_with_value.wren b/test/fiber/run_with_value.wren
index 9a201518..e3cef529 100644
--- a/test/fiber/run_with_value.wren
+++ b/test/fiber/run_with_value.wren
@@ -4,6 +4,8 @@ var fiber = new Fiber {
 
 // The first value passed to the fiber is ignored, since there's no yield call
 // to return it.
-IO.print("before")   // expect: before
+IO.print("before")    // expect: before
 fiber.run("ignored") // expect: fiber
-IO.print("after")    // expect: after
+
+// This does not get run since we exit when the run fiber completes.
+IO.print("nope")
diff --git a/test/fiber/run_with_value_direct_reenter.wren b/test/fiber/run_with_value_direct_reenter.wren
new file mode 100644
index 00000000..dead5b28
--- /dev/null
+++ b/test/fiber/run_with_value_direct_reenter.wren
@@ -0,0 +1,10 @@
+var fiber
+
+fiber = new Fiber {
+  IO.print(1) // expect: 1
+  fiber.run("ignored")
+  IO.print(2) // expect: 2
+}
+
+fiber.call
+IO.print(3) // expect: 3
\ No newline at end of file
diff --git a/test/fiber/run_with_value_indirect_reenter.wren b/test/fiber/run_with_value_indirect_reenter.wren
new file mode 100644
index 00000000..2387ce3e
--- /dev/null
+++ b/test/fiber/run_with_value_indirect_reenter.wren
@@ -0,0 +1,20 @@
+var a
+var b
+
+a = new Fiber {
+  IO.print(2)
+  b.run("ignored")
+  IO.print("nope")
+}
+
+b = new Fiber {
+  IO.print(1)
+  a.run("ignored")
+  IO.print(3)
+}
+
+b.call
+// expect: 1
+// expect: 2
+// expect: 3
+IO.print(4) // expect: 4
diff --git a/test/fiber/run_with_value_when_done.wren b/test/fiber/run_with_value_when_done.wren
deleted file mode 100644
index fbcd1d8c..00000000
--- a/test/fiber/run_with_value_when_done.wren
+++ /dev/null
@@ -1,6 +0,0 @@
-var fiber = new Fiber {
-  IO.print("run")
-}
-
-fiber.run(1) // expect: run
-fiber.run(2) // expect runtime error: Cannot run a finished fiber.
diff --git a/test/fiber/yield.wren b/test/fiber/yield.wren
index 5f2c0992..c262a39e 100644
--- a/test/fiber/yield.wren
+++ b/test/fiber/yield.wren
@@ -6,9 +6,9 @@ var fiber = new Fiber {
   IO.print("fiber 3")
 }
 
-var result = fiber.run // expect: fiber 1
-IO.print("main 1") // expect: main 1
-result = fiber.run // expect: fiber 2
-IO.print("main 2") // expect: main 2
-result = fiber.run // expect: fiber 3
-IO.print("main 3") // expect: main 3
+var result = fiber.call // expect: fiber 1
+IO.print("main 1")  // expect: main 1
+result = fiber.call // expect: fiber 2
+IO.print("main 2")  // expect: main 2
+result = fiber.call // expect: fiber 3
+IO.print("main 3")  // expect: main 3
diff --git a/test/fiber/yield_return_value.wren b/test/fiber/yield_return_call_value.wren
similarity index 72%
rename from test/fiber/yield_return_value.wren
rename to test/fiber/yield_return_call_value.wren
index 3314a31e..ce6b0bf1 100644
--- a/test/fiber/yield_return_value.wren
+++ b/test/fiber/yield_return_call_value.wren
@@ -6,9 +6,9 @@ var fiber = new Fiber {
   IO.print(result)
 }
 
-fiber.run // expect: fiber 1
+fiber.call // expect: fiber 1
 IO.print("main 1") // expect: main 1
-fiber.run("run 1") // expect: run 1
+fiber.call("call 1") // expect: call 1
 IO.print("main 2") // expect: main 2
-fiber.run // expect: null
+fiber.call // expect: null
 IO.print("main 3") // expect: main 3
diff --git a/test/fiber/yield_return_run_value.wren b/test/fiber/yield_return_run_value.wren
new file mode 100644
index 00000000..3c951da7
--- /dev/null
+++ b/test/fiber/yield_return_run_value.wren
@@ -0,0 +1,12 @@
+var fiber = new Fiber {
+  IO.print("fiber")
+  var result = Fiber.yield
+  IO.print(result)
+}
+
+fiber.call // expect: fiber
+IO.print("main") // expect: main
+fiber.run("run") // expect: run
+
+// This does not get run since we exit when the run fiber completes.
+IO.print("nope")
diff --git a/test/fiber/yield_with_no_caller.wren b/test/fiber/yield_with_no_caller.wren
new file mode 100644
index 00000000..ccbeeae6
--- /dev/null
+++ b/test/fiber/yield_with_no_caller.wren
@@ -0,0 +1,9 @@
+var a = new Fiber {
+  Fiber.yield // expect runtime error: No fiber to yield to.
+}
+
+// Run a chain of fibers. Since none of them are called, they all get discarded
+// and there is no remaining caller.
+var b = new Fiber { a.run }
+var c = new Fiber { b.run }
+c.run
diff --git a/test/fiber/yield_with_value.wren b/test/fiber/yield_with_value.wren
index 9ed579c0..e7d8d9a1 100644
--- a/test/fiber/yield_with_value.wren
+++ b/test/fiber/yield_with_value.wren
@@ -6,9 +6,9 @@ var fiber = new Fiber {
   IO.print("fiber 3")
 }
 
-var result = fiber.run // expect: fiber 1
+var result = fiber.call // expect: fiber 1
 IO.print(result) // expect: yield 1
-result = fiber.run // expect: fiber 2
+result = fiber.call // expect: fiber 2
 IO.print(result) // expect: yield 2
-result = fiber.run // expect: fiber 3
+result = fiber.call // expect: fiber 3
 IO.print(result) // expect: null
diff --git a/test/fiber/yield_with_value_with_no_caller.wren b/test/fiber/yield_with_value_with_no_caller.wren
new file mode 100644
index 00000000..591eef64
--- /dev/null
+++ b/test/fiber/yield_with_value_with_no_caller.wren
@@ -0,0 +1,9 @@
+var a = new Fiber {
+  Fiber.yield(1) // expect runtime error: No fiber to yield to.
+}
+
+// Run a chain of fibers. Since none of them are called, they all get discarded
+// and there is no remaining caller.
+var b = new Fiber { a.run }
+var c = new Fiber { b.run }
+c.run