The CRP mutant of Salmonella choleraesuis smith was constructed by the allelic exchange method with pRE112 suicide plasmid. Firstly, the upstream and downstream fragments of CRP gene were amplified from Salmonella choleraesuis smith genome by pwo-polymerase and then successively subcloned into cloning vector pBluescript II SK+ to construct the recombinant vector pBSIIK+/ CRP with 322 bp deletion mutant The CRP was then transfered to suicide plasmid pRE1l2 and introduced into Salmonella choleraesuis Smith genome through the two-step recombination method. The O and H antigenes of the mutant was 0: 6.7 H: 1.5, identical to the parent strain. The mutant was stable with the recombinant CRP gene through 20 continued generations. However, the mutant lost fermentation ability with almost sugars, except glucose, and grew more slowly than the parent S. choleraesuis smith strain. These results showed that the S. choleraesuis smith CRP mutant was constructed succesfully.