Hi, I think there's a bug in leastsq when Jacobian is involved -- the full_output option has to be 1 if Dfun is assigned to a user-defined function, otherwise there's a segmentation fault. gdb shows the seg fault happens here: Program received signal SIGSEGV, Segmentation fault. [Switching to Thread -1210287456 (LWP 17643)] 0xb7e2c01a in malloc_usable_size () from /lib/tls/libc.so.6 If I force not to use the thread-local libc, I still got a seg fault: Program received signal SIGSEGV, Segmentation fault. [Switching to Thread 16384 (LWP 17610)] 0xb7e7f1aa in __libc_malloc_pthread_startup () from /lib/libc.so.6 This problem doesn't happen if Jocobian (Dfun) is None. It also doesn't happen when running on Debian x86_64 version. I'm using scipy 0.5.3, Debian i686 2.6.18 kernel. Thanks! --lin