The sequence homology of clindamycin resistance (Clnr) determinants was studied in 16 Clnr Bacteroides strains. The isolates were surveyed for plasmid content, homology with the Clnr determinant of pBFTM10, and ability to transfer Clnr. The Clnr DNA probes used in the Southern hybridizations were pBFTM10 and a plasmid derivative containing an EcoRI fragment of pBFTM10 cloned into Escherichia coli. A total of 13 of 16 Clnr strains also carried tetracycline resistance, and 15 of 16 Clnr Bacteroides isolates showed homology with the Clnr determinant of pBFTM10. These data suggest that the previously characterized Clnr determinant of pBFTM10 is widely distributed in nature and may be found on either a plasmid or the chromosome. The Clnr Bacteroides fragilis strain which lacked homology with pBFTM10 also had different transfer properties; thus, more than one type of Clnr determinant may exist in Bacteroides spp.