Polychlorinated biphenyl (PCB) transformation activity of a strong PCB degrader, Rhodococcus sp. strain RHA1, was examined in different concentrations of PCBs. A extremely strong PCB transformation activity was observed on 30 mu g PCB/ml. At 50 and 100 mu g/ml, transformation activities were diminished. In the case of bphA insertion mutant, RDA1, transformation activity in the presence of ethylbezene was poor even at 30 mu g/ml. This indicated that the bphA dependent system would play a major role in PCB transformation by RHA1. Greater transformation activity of RHA1 was observed in the presence of ethylbenzene than in the presence of biphenyl.