Rhodococcus erythropolis TA421, a polychlorinated biphenyl and biphenyl degrader isolated from a termiteecosystem, has seven bphC genes expressing 2,3-dihydroxybiphenyl dioxygenase activity. R. erythropolis TA421harbored a large and probably linear plasmid on which three (bphC2, bphC3, and bphC4) of the seven bphCgenes were located. A non-biphenyl-degrading mutant, designated strain TA422, was obtained spontaneouslyfrom R. erythropolis TA421. TA422 lacked the plasmid, suggesting that the three bphC genes were involved inthe degradation of biphenyl. Southern blot analyses showed that R. erythropolis TA421 and Rhodococcusgloberulus P6 have a similar set of bphC genes and that the genes for biphenyl catabolism are located onplasmids of different sizes. These results indicated that the genes encoding the biphenyl catabolic pathway inRhodococcus strains are borne on plasmids.