diff --git a/mitogen/core.py b/mitogen/core.py
index 2769021e..c6220d64 100644
--- a/mitogen/core.py
+++ b/mitogen/core.py
@@ -162,6 +162,11 @@ def set_nonblock(fd):
     fcntl.fcntl(fd, fcntl.F_SETFL, flags | os.O_NONBLOCK)
 
 
+def set_block(fd):
+    flags = fcntl.fcntl(fd, fcntl.F_GETFL)
+    fcntl.fcntl(fd, fcntl.F_SETFL, flags & ~os.O_NONBLOCK)
+
+
 def io_op(func, *args):
     try:
         return func(*args), False
diff --git a/mitogen/master.py b/mitogen/master.py
index 37829e41..2e30eaf4 100644
--- a/mitogen/master.py
+++ b/mitogen/master.py
@@ -102,6 +102,7 @@ def create_child(*args):
     parentfp, childfp = socket.socketpair()
     pid = os.fork()
     if not pid:
+        mitogen.core.set_block(childfp.fileno())
         os.dup2(childfp.fileno(), 0)
         os.dup2(childfp.fileno(), 1)
         childfp.close()