The relations between growth rate (μ, h-1) and organic nutrient concentrations (S, mg/l) were examined by using natural bacterial populations and isolated strains in Lake Biwa. The kinetic parameters Ks (half-saturation constant) of three groups of natural populations counted with different agar media containing 0.5, 5 and 10 g peptone per liter were 1.15, 0.82 and 0.49 mg/l, respectively. The Ks of these three groups of bacteria isolated were <0.005-6.94, 0.04-313 and 0.156-4000 mg/l, respectively.The relations between the colony number and the concentration of peptone in agar medium were also examined for different size fractions of suspended particles in the lake water.According to the results obtained, the micro-habitat segregation of natural bacteria in the lake water was discussed.