Abstract The precipitation mechanisms of Mn(II) were studied from the viewpoint of adsorption onto suspended solids (SS) and microbial oxidation of Mn in Lake Biwa. In aerobic water samples maintained in the laboratory, the concentration of dissolved Mn [Mn(II)] decreased at pH > 7 by microbially mediated oxidation. Filtration, autoclaving and irradiation of u.v. light of the water samples and the addition of NaN 3 inhibited the precipitation of Mn(II). The adsorption of Mn(II) onto SS was also appreciable at pH > 7, and attained equilibrium within at least 30 min. The oxidation rate of Mn(II) was much less than the adsorption rate. In various environments of the lake (bottom water, sediment surface, river mouth, etc.) the initial adsorption of Mn(II) and subsequent slow oxidation mediated by bacteria may be essential to the mechanisms of Mn(II) precipitation.