The condition of hydrogen sulfide production in bottom sediments was investigated in the mesotrophic Northern and eutrophic Southern Lake Biwa by using radioactive sulfa te (³⁵SO₄^2-) experiment. The concentration of total sulfides in the surface sediment was much higher in the Northern Lake than in the Southern Lake. This higher concentration of total sulfides in the Nor-them Lake was considered to be the result of the activity of sulfate reduction (20 nmol/cm³/day), which was five times higher than that in the Southern Lake. The activity of sulfate reduction by bacterla was not necessarily correlated with the number of sulfate reducing bacteria in Lake Biwa. Lower redox potential and higher concentration of organic acids were of greater advantage for sulfide production in the Northern Lake than in the Southern Lake. The organic acids which are utilized as the electron donor in sulfate reduction were butyrate and formate in this lake sedi-ment, and acetate present in hieh amount was not utilized.