diff --git a/README.md b/README.md
index 6d8bdbe..cf5420c 100644
--- a/README.md
+++ b/README.md
@@ -115,6 +115,15 @@ argument to see all DeepState options.
 
 DeepState consists of a static library, used to write test harnesses, and command-line _executors_ written in Python. At this time, the best documentation is in the [examples](/examples) and in our [paper](https://agroce.github.io/bar18.pdf).  A more extensive example, using DeepState and libFuzzer to test a user-mode file system, is available [here](https://github.com/agroce/testfs); in particular the [Tests.cpp](https://github.com/agroce/testfs/blob/master/Tests.cpp) file and CMakeLists.txt show DeepState usage.
 
+## A Note on Mac OS and Forking
+
+Normally, when running a test for replay or fuzzing, DeepState forks
+in order to cleanly handle crashes of a test.  Unfortunately, `fork()`
+on mac OS is extremely slow.  When using the built-in fuzzer or
+replaying tests, it is highly recommended to add the `--no_fork`
+option on mac OS, unless you need the added crash handling (that is,
+things aren't working without that option).
+
 ## Built-In Fuzzer
 
 Every DeepState executable provides a simple built-in fuzzer that
diff --git a/src/include/deepstate/DeepState.h b/src/include/deepstate/DeepState.h
index 163d921..51c3cee 100644
--- a/src/include/deepstate/DeepState.h
+++ b/src/include/deepstate/DeepState.h
@@ -66,6 +66,7 @@ DECLARE_bool(abort_on_fail);
 DECLARE_bool(verbose_reads);
 DECLARE_bool(fuzz);
 DECLARE_bool(fuzz_save_passing);
+DECLARE_bool(no_fork);
 
 DECLARE_int(log_level);
 DECLARE_int(seed);
@@ -533,7 +534,7 @@ static void DeepState_RunTest(struct DeepState_TestInfo *test) {
 }
 
 /* Run a test case, but in libFuzzer, so not inside a fork. */
-static int DeepState_RunTestLLVM(struct DeepState_TestInfo *test) {
+static int DeepState_RunTestNoFork(struct DeepState_TestInfo *test) {
   /* Run the test. */
   if (!setjmp(DeepState_ReturnToRun)) {
     /* Convert uncaught C++ exceptions into a test failure. */
@@ -577,13 +578,20 @@ static int DeepState_RunTestLLVM(struct DeepState_TestInfo *test) {
 /* Fork and run `test`. */
 static enum DeepState_TestRunResult
 DeepState_ForkAndRunTest(struct DeepState_TestInfo *test) {
-  pid_t test_pid = fork();
-  if (!test_pid) {
-    DeepState_RunTest(test);
+  pid_t test_pid;
+  if (!FLAGS_no_fork) {
+    test_pid = fork();
+    if (!test_pid) {
+      DeepState_RunTest(test);
+    }
   }
   int wstatus;
-  waitpid(test_pid, &wstatus, 0);
-
+  if (!FLAGS_no_fork) {
+    waitpid(test_pid, &wstatus, 0);
+  } else {
+    wstatus = DeepState_RunTestNoFork(test);
+  }
+  
   /* If we exited normally, the status code tells us if the test passed. */
   if (WIFEXITED(wstatus)) {
     uint8_t status = WEXITSTATUS(wstatus);
diff --git a/src/lib/DeepState.c b/src/lib/DeepState.c
index b35c2b8..cfd1657 100644
--- a/src/lib/DeepState.c
+++ b/src/lib/DeepState.c
@@ -39,6 +39,7 @@ DEFINE_bool(abort_on_fail, false, "Abort on file replay failure (useful in file
 DEFINE_bool(verbose_reads, false, "Report on bytes being read during execution of test.");
 DEFINE_bool(fuzz, false, "Perform brute force unguided fuzzing.");
 DEFINE_bool(fuzz_save_passing, false, "Save passing tests during fuzzing.");
+DEFINE_bool(no_fork, false, "Don't fork when running a test.");
 
 DEFINE_int(log_level, 0, "Minimum level of logging to output.");
 DEFINE_int(seed, 0, "Seed for brute force fuzzing (uses time if not set).");
@@ -660,7 +661,7 @@ extern int LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size) {
 
   DeepState_Begin(test);
 
-  enum DeepState_TestRunResult result = DeepState_RunTestLLVM(test);
+  enum DeepState_TestRunResult result = DeepState_RunTestNoFork(test);
 
   const char* abort_check = getenv("LIBFUZZER_ABORT_ON_FAIL");
   if (abort_check != NULL) {