Mercury resistance of Clostridium cochlearium T-2P was found to be controlled by a different mechanism from those reported so far since no mercury-reducing activity was detected in this strain. The H2S generating ability as well as the demethylating activity in this bacterium was eliminated by the treatment of the cured acridine dye and recovered by the conjugation of the cured strain with the parent strain. In addition, the strain which lost their abilities to generate H2S and to decompose methylmercury, showed higher sensitivity to mercurials than the parent strain. From these results, the genes conferring both the activities seemed to reside on the plasmid and the mechanism of mercury resistance was probably based on a detoxification mechanism involving methylmercury decomposition and inactivation of the inorganic mercury with H2S.