The genetic basis of clindamycin and tetracycline resistance in species of Bacteroides was studied in 13 resistant clinical isolates; 10 isolates were resistant to high levels of clindamycin (greater than 160 micrograms/ml) and three were resistant to low levels of clindamycin (5-20 micrograms/ml). All of the isolates were resistant to tetracycline. None of the strains with low-level clindamycin resistance could transfer resistance to either antibiotic. Three isolates resistant to high levels of clindamycin transferred tetracycline resistance to a sensitive Bacteroides fragilis recipient, and one of these (strain 1126) also transferred high-level clindamycin resistance. None of the transfer-proficient strains of Bacteroides contained plasmid DNA. However, DNA homology was detected by hybridization between the clindamycin resistance plasmid pBF4 isolated in France and sequences in our high-level clindamycin-resistant strains from California. A common homologous 4.8 megadalton EcoR1 fragment was identified in the whole cell DNA of the transfer-proficient strain 1126 of B. fragilis and two other species of the B. fragilis group.