A mer operon of mercury-resistant Pseudoalteromonas haloplanktis strain M1, isolated from sea water of Minamata Bay, was cloned and analyzed. The mer genes were located in the chromosome and organized as merR-merT-merP-merC-merA-merD, the same order as that in Tn21. However, the orientation of the merR gene is the same as that of other mer genes (opposite direction to Tn21), and merR was cotranscribed with other mer genes, a pattern that has not been previously seen with mer determinants from other Gram-negative bacteria. Furthermore, the amino acid similarities of the corresponding mer gene products between those from strain M1 and Tn21 were unusually low.