using openMPI on OS X 10.5

[John Grefenstette]

I am using gcc version 4.0.1 on a MacPro dual-core intel
machine running OS X 10.5.4.

I have OpenMPI v1.2.3 installed.  When I compile, I get
the message:

% gcc  -fopenmp -O3  -DUNIX xxx.c -c
cc1: error: unrecognized command line option "-fopenmp"

Any help appreciated.

John