diff --git a/src/libstd/rt/comm.rs b/src/libstd/rt/comm.rs
index 42d59ccdf958e..49cf8c239b7e6 100644
--- a/src/libstd/rt/comm.rs
+++ b/src/libstd/rt/comm.rs
@@ -727,6 +727,7 @@ mod test {
     use rt::test::*;
     use cell::Cell;
     use iter::Times;
+    use rt::util;
 
     #[test]
     fn oneshot_single_thread_close_port_first() {
@@ -875,6 +876,7 @@ mod test {
 
     #[test]
     fn oneshot_multi_thread_close_stress() {
+        if util::limit_thread_creation_due_to_osx_and_valgrind() { return; }
         do stress_factor().times {
             do run_in_newsched_task {
                 let (port, chan) = oneshot::<int>();
@@ -890,6 +892,7 @@ mod test {
 
     #[test]
     fn oneshot_multi_thread_send_close_stress() {
+        if util::limit_thread_creation_due_to_osx_and_valgrind() { return; }
         do stress_factor().times {
             do run_in_newsched_task {
                 let (port, chan) = oneshot::<int>();
@@ -910,6 +913,7 @@ mod test {
 
     #[test]
     fn oneshot_multi_thread_recv_close_stress() {
+        if util::limit_thread_creation_due_to_osx_and_valgrind() { return; }
         do stress_factor().times {
             do run_in_newsched_task {
                 let (port, chan) = oneshot::<int>();
@@ -936,6 +940,7 @@ mod test {
 
     #[test]
     fn oneshot_multi_thread_send_recv_stress() {
+        if util::limit_thread_creation_due_to_osx_and_valgrind() { return; }
         do stress_factor().times {
             do run_in_newsched_task {
                 let (port, chan) = oneshot::<~int>();
@@ -955,6 +960,7 @@ mod test {
 
     #[test]
     fn stream_send_recv_stress() {
+        if util::limit_thread_creation_due_to_osx_and_valgrind() { return; }
         do stress_factor().times {
             do run_in_mt_newsched_task {
                 let (port, chan) = stream::<~int>();
@@ -999,6 +1005,7 @@ mod test {
 
     #[test]
     fn shared_chan_stress() {
+        if util::limit_thread_creation_due_to_osx_and_valgrind() { return; }
         do run_in_mt_newsched_task {
             let (port, chan) = stream();
             let chan = SharedChan::new(chan);
@@ -1018,6 +1025,7 @@ mod test {
 
     #[test]
     fn shared_port_stress() {
+        if util::limit_thread_creation_due_to_osx_and_valgrind() { return; }
         do run_in_mt_newsched_task {
             // XXX: Removing these type annotations causes an ICE
             let (end_port, end_chan) = stream::<()>();
@@ -1098,6 +1106,8 @@ mod test {
         use rand;
         use rand::RngUtil;
 
+        if util::limit_thread_creation_due_to_osx_and_valgrind() { return; }
+
         do run_in_mt_newsched_task {
             let (end_port, end_chan) = stream::<()>();
             let end_chan = SharedChan::new(end_chan);
diff --git a/src/libstd/rt/sched.rs b/src/libstd/rt/sched.rs
index ce4e64c47d2ef..e65a45f0e0749 100644
--- a/src/libstd/rt/sched.rs
+++ b/src/libstd/rt/sched.rs
@@ -819,6 +819,7 @@ mod test {
     use cell::Cell;
     use rt::thread::Thread;
     use rt::task::{Task, Sched};
+    use rt::util;
     use option::{Some};
 
     #[test]
@@ -1040,6 +1041,7 @@ mod test {
 
     #[test]
     fn test_stress_schedule_task_states() {
+        if util::limit_thread_creation_due_to_osx_and_valgrind() { return; }
         let n = stress_factor() * 120;
         for _ in range(0, n as int) {
             test_schedule_home_states();
diff --git a/src/libstd/rt/test.rs b/src/libstd/rt/test.rs
index a933115774933..4e2f9fbe13075 100644
--- a/src/libstd/rt/test.rs
+++ b/src/libstd/rt/test.rs
@@ -18,7 +18,7 @@ use iterator::{Iterator, range};
 use super::io::net::ip::{SocketAddr, Ipv4Addr, Ipv6Addr};
 use vec::{OwnedVector, MutableVector, ImmutableVector};
 use rt::sched::Scheduler;
-use unstable::run_in_bare_thread;
+use unstable::{run_in_bare_thread};
 use rt::thread::Thread;
 use rt::task::Task;
 use rt::uv::uvio::UvEventLoop;
@@ -162,10 +162,14 @@ pub fn run_in_mt_newsched_task(f: ~fn()) {
         let nthreads = match os::getenv("RUST_RT_TEST_THREADS") {
             Some(nstr) => FromStr::from_str(nstr).unwrap(),
             None => {
-                // Using more threads than cores in test code
-                // to force the OS to preempt them frequently.
-                // Assuming that this help stress test concurrent types.
-                util::num_cpus() * 2
+                if util::limit_thread_creation_due_to_osx_and_valgrind() {
+                    1
+                } else {
+                    // Using more threads than cores in test code
+                    // to force the OS to preempt them frequently.
+                    // Assuming that this help stress test concurrent types.
+                    util::num_cpus() * 2
+                }
             }
         };
 
diff --git a/src/libstd/rt/util.rs b/src/libstd/rt/util.rs
index b8c7c8761e85d..c81f3ec9a7993 100644
--- a/src/libstd/rt/util.rs
+++ b/src/libstd/rt/util.rs
@@ -15,6 +15,9 @@ use option::{Some, None};
 use os;
 use str::StrSlice;
 
+#[cfg(target_os="macos")]
+use unstable::running_on_valgrind;
+
 /// Get the number of cores available
 pub fn num_cpus() -> uint {
     #[fixed_stack_segment]; #[inline(never)];
@@ -28,12 +31,35 @@ pub fn num_cpus() -> uint {
     }
 }
 
+/// Valgrind has a fixed-sized array (size around 2000) of segment descriptors wired into it; this
+/// is a hard limit and requires rebuilding valgrind if you want to go beyond it. Normally this is
+/// not a problem, but in some tests, we produce a lot of threads casually. Making lots of threads
+/// alone might not be a problem _either_, except on OSX, the segments produced for new threads
+/// _take a while_ to get reclaimed by the OS. Combined with the fact that libuv schedulers fork off
+/// a separate thread for polling fsevents on OSX, we get a perfect storm of creating "too many
+/// mappings" for valgrind to handle when running certain stress tests in the runtime.
+#[cfg(target_os="macos")]
+pub fn limit_thread_creation_due_to_osx_and_valgrind() -> bool {
+    running_on_valgrind()
+}
+
+#[cfg(not(target_os="macos"))]
+pub fn limit_thread_creation_due_to_osx_and_valgrind() -> bool {
+    false
+}
+
 /// Get's the number of scheduler threads requested by the environment
 /// either `RUST_THREADS` or `num_cpus`.
 pub fn default_sched_threads() -> uint {
     match os::getenv("RUST_THREADS") {
         Some(nstr) => FromStr::from_str(nstr).unwrap(),
-        None => num_cpus()
+        None => {
+            if limit_thread_creation_due_to_osx_and_valgrind() {
+                1
+            } else {
+                num_cpus()
+            }
+        }
     }
 }
 
diff --git a/src/libstd/run.rs b/src/libstd/run.rs
index a43d448dae51f..eeaee4c14a512 100644
--- a/src/libstd/run.rs
+++ b/src/libstd/run.rs
@@ -955,6 +955,7 @@ mod tests {
     use path::Path;
     use run;
     use str;
+    use unstable::running_on_valgrind;
 
     #[test]
     #[cfg(windows)]
@@ -1365,13 +1366,4 @@ mod tests {
 
         assert!(output.contains("RUN_TEST_NEW_ENV=123"));
     }
-
-    fn running_on_valgrind() -> bool {
-        #[fixed_stack_segment]; #[inline(never)];
-        unsafe { rust_running_on_valgrind() != 0 }
-    }
-
-    extern {
-        fn rust_running_on_valgrind() -> uintptr_t;
-    }
 }
diff --git a/src/libstd/unstable/mod.rs b/src/libstd/unstable/mod.rs
index 6ad15bfc7c039..51de3caf2aebb 100644
--- a/src/libstd/unstable/mod.rs
+++ b/src/libstd/unstable/mod.rs
@@ -14,6 +14,7 @@ use comm::{GenericChan, GenericPort};
 use comm;
 use prelude::*;
 use task;
+use libc::uintptr_t;
 
 pub mod dynamic_lib;
 
@@ -118,3 +119,17 @@ pub fn change_dir_locked(p: &Path, action: &fn()) -> bool {
         fn rust_drop_change_dir_lock();
     }
 }
+
+
+/// Dynamically inquire about whether we're running under V.
+/// You should usually not use this unless your test definitely
+/// can't run correctly un-altered. Valgrind is there to help
+/// you notice weirdness in normal, un-doctored code paths!
+pub fn running_on_valgrind() -> bool {
+    #[fixed_stack_segment]; #[inline(never)];
+    unsafe { rust_running_on_valgrind() != 0 }
+}
+
+extern {
+    fn rust_running_on_valgrind() -> uintptr_t;
+}