Thimerosal (ThM) resistant bacteria isolated from the scum of discarded vaccines containing ThM were identified as Pseudomonas sp. on the basis of its bacteriological properties. The isolated bacteria were resistant to 1165 μg/ml of ThM and also to 200 μg/ml of mercuric chloride (MC). By incubation of a mixture of ThM and the bacteria, mercury decreased from the supernatant of the culture. ThM incorporated by the bacteria could neither be dialyzed by cysteine equilibrium dialysis nor be detected by a dithizone method. After formalin treatment, the bacteria could not incorporate ThM. ThM, therefore, may be incorporated actively by the bacteria. In the case of MC, however, most of MC seems to be adsorbed on the cell surface immediately after its addition in contrast to ThM. Then incorporared ThM might be localized near the surface of the cell wall, and then enzymatically decomposed to vaporizable form, since the total mercury decreased in the reaction mixture with an increase in inorganic mercury. When sodium selenite (Se), equivalent to ThM, was added to the reaction mixture containing ThM and Pseudomonas sp. and the mixture was incubated at 27°C, total mercury in the reaction mixture did not decrease and inorganic mercury could not be detected in the suspension. It was presumed that Se may bind to inorganic mercury in the existence of cell components, and that it may prevent from vaporizing mercury from the suspension.