The sulfate reduction rate was measured for almost four years in the profundal sediments of Lake Kizaki, a mesotrophic lake in central Japan. The rate was generally highest in the surface layer and decreased with depth. Seasonally, sulfate reduction tended to be high in spring and summer, and then to decrease until the end of stratification (December) in spite of a constant in situ temperature of around 6 "C, although fluctuations were found in every year. The rate also fluctuated greatly according to year. The maximum rate of sulfate reduction was 0.33 mmol mm2 d-l in May, 1990, and the minimum was 0.004 mmol m-' d-l in March, 1993. These relatively low rates, compared with those reported for freshwater sediments, seem to be due to low concentrations of sulfate in the sediments (5-23 pmol 1-l in the surface layer) . The rate was highly correlated with the concentration of sulfate in the sediments. The addition of sulfate stimulated sulfate reduction in all sediment samples tested, but adding lactate did not. Therefore, sulfate reduction should be limited mainly by the supply of sulfate. Measurements of sulfate reduction rates at different concentrations of added sulfate revealed a low concentration of half-saturation constant as low as 12 pm01 1-l.