We found that cells of Vibrio parahaemolyticus possess an energy-dependent efflux system for norfloxacin. We cloned a gene for a putative norfloxacin efflux protein from the chromosomal DNA of V. parahaemolyticus by using an Escherichia coli mutant lacking the major multidrug efflux system AcrAB as the host and sequenced the gene (norM). Cells of E. coli transformed with a plasmid carrying the norM gene showed elevated energy-dependent efflux of norfloxacin. The transformants showed elevated resistance not only to norfloxacin and ciprofloxacin but also to the structurally unrelated compounds ethidium, kanamycin, and streptomycin. These results suggest that this is a multidrug efflux system. The hydropathy pattern of the deduced amino acid sequence of NorM suggested the presence of 12 transmembrane domains. The deduced primary structure of NorM showed 57% identity and 88% similarity with that of a hypothetical E. coli membrane protein, YdhE. No reported drug efflux protein in the sequence databases showed significant sequence similarity with NorM. Thus, NorM seems to be a novel type of multidrug efflux protein. We cloned the ydhE gene from E. coli. Cells of E. coli transformed with the cloned ydhE gene showed elevated resistance to norfloxacin, ciprofloxacin, acriflavine, and tetraphenylphosphonium ion, but not to ethidium, when MICs were measured. Thus, it seems that NorM and YdhE differ somehow in substrate specificity.