The chloramphenicol resistance determinant from a nonconjugative strain of Clostridium perfringens was cloned and shown to be expressed in Escherichia coli. Subcloning and deletion analysis localized the resistance gene, catQ, to within a 1.25-kilobase (kb) partial Sau3A fragment. The catQ gene contained internal HindII, HaeIII, and DRaI restriction sites and was distinct from the catP gene, which was originally cloned (L.J. Abraham, A.J. Wales, and J.I. Rood, Plasmid 14:37-46, 1985) from the conjugative C. perfringens R plasmid, pIP401. Hybridization studies were carried out with a 0.35-kb DraI-PstI fragment of pJIR260 as an internal catQ-specific probe and a 0.38-kb EcoRV-HinfI fragment of pJIR62 as an internal catP-specific gene probe. The results showed that the catP and catQ genes were not similar and that neither probe hybridized with cat genes from the other bacterial genera. However, the catP gene was similar to the cloned catD gene from Clostridium difficile. Comparative studies with both catP and catD probes showed that these genes had significant restriction identity. We therefore suggest that these genes were derived from a common source.