The rates of anaerobic respirations, methanogenesis, sulfate reduction, and denitrification were studied for sediments of three sites along the watercourse in the polluted lower Tama River, Japan, in summer and winter. The rate of methanogenesis was high at the upper freshwater site (Site 1) and low in lower sites (Sites 2 and 3) in accordance to the level of sulfate in interstitial water. On the contrary, the rate of sulfate reduction was high at the lower sites. Denitrification was high at Site 1 and the intermediate site (Site 2). The relative importance of anaerobic respirations was estimated on area basis. At Site 1, methanogenesis was the dominant process in summer, but it was slightly lower than denitrification in winter. At Sites 2 and 3, the dominant process was denitrification and sulfate reduction, respectively, in both seasons.