The erythromycin resistance determinant from Clostridium perfringens CP592 was cloned and shown to be expressed in Escherichia coli. The resultant plasmid, pJIR122 (7.9 kilobase pairs [kb]), was unstable since in both recA+ and recA E. coli hosts spontaneous deletion of 2.7 kb, including the erythromycin resistance determinant, was observed. Subcloning, as well as deletion analysis with BAL 31, localized the erythromycin resistance gene (ermP) to within a 1.0-kb region of pJIR122. A 0.5-kb fragment internal to ermP was 32P labeled and used as an ermP-specific probe in DNA hybridization experiments which used target DNA prepared from representatives of each of the known erm classes and also from erythromycin-resistant isolates of a variety of clostridial species. Hybridizing sequences were detected in DNA from several Clostridium difficile isolates and a Clostridium paraputrificum strain; however, ermP was not widespread in erythromycin-resistant C. perfringens isolates. The ermP determinant hybridized to, and shared significant restriction identity with, the ermB class gene from the streptococcal plasmid pAM beta 1. No hybridization was detected with the other six hybridization classes of erm determinants.