A strain of Moraxella sp. capable of assimilating fluoroacetate (FA) as the sole source of carbon was isolated. It was resistant to mercuric chloride and antibiotics, such as penicillin G and kanamycin. Mercury reductase and haloacetate halidohydrolase were found in the cell-free extract of the organism. Two kinds of halidohydrolase were found on purification by DEAE-cellulose column chromatography; one was active for FA and chloroacetate (CA) (halidohydrolase 1), and the other was active for CA but not FA (halidohydrolase 2). Two types of cured strain were obtained by mitomycin C; type I lost halidohydrolase 2, whereas type II became sensitive to mercuric chloride and lost halidohydrolase 1 and 2. On electrophoresis of plasmid on agarose gel, a plasmid band was observed in a wild strain and a shorter band in type I. However, no band was observed in type II. Transferring of the plasmid by conjugation from a wild cell to a type II cell gave an exconjugant which possessed halidohydrolase 1 and 2 and became resistant to mercury. These results indicate that both halidohydrolase 1 and 2 and also mercury reductase are determined by the plasmid in the organism.