Bacterial flora in 6 species of bivalves, Tapes philippinarum, Mactra veneriformis, Mitylis coruscus, Crassostrea gigas, Phacosoma japonicum and Scapharca broughtonii, sediment and seawater in Tokyo bay were investigated by using PYBG, 1/20 PYBG, N-PYBG, GAM, DHL, LBS and PEA agar media under aerobic and anaerobic conditions. Different values of colony counts were observed on 7 different media. Major aerobic bacteria of bivalves were composed of Vibrio-Aeromonas, Pseudomonas, Enterobacteriaceae, Moraxella and Micrococcus. Anaerobic bacteria belonged to Bacteroidaceae and clostridia, which were minor components. Bacterial components of sediment and seawater were similar to those of bivalves. These results suggest that bacterial flora of bivalves are well influenced by environmental flora.