This bug needs an answer from someone with unix expertise: <http://www.twistedmatrix.com/users/itamarst.twistd/bugs/view_bug?bug_id=8> I suggest getting rid of SIGCHLD handler, and having t.i.process.Process instances keep track of the pid of their process, and pass it to reapProcess so it only waitpid()s on a specific pid, and not all of them. I will add a test case which will test for this bug.