A chromate-resistant strain of Pseudomonas mendocina MCM B-180 capable of reducing hexavalent chromium was found to harbour a single plasmid. Incubation of the strain at 42 degrees C for 24 h caused loss of chromate resistance as well as the plasmid, pARI180. Transformation of E. coli DH5 alpha with purified pARI180 plasmid DNA resulted in simultaneous acquisition of resistance to chromate and the appearance of plasmid in the transformants. Most importantly, the plasmid transfer was found to confer chromate reduction ability on to the E. coli transformants.