In oligotrophic waters, not only community structure but also physiological properties of heterotrophic bacteria are influenced by the concentration of organic matter.The relationship between growth rate of two facultatively oligotrophic strains ofAeromonas sp. No. 6 andFlavobacterium sp. M1 was studied in comparison with that of two eutrophic strains ofEscherichia coli 7020 andFlavobacterium sp. M2. These strains had two or three different substrate constants (Ks values) depending on substrate concentrations: Ks values for the two former were remarkably lower than those for the two latter. For instance, Ks value forAeromonas sp. No. 6 was about 8.9μM when substrate concentration was greater than 53μM and about 1.1μM when substrate concentration was less man 53μM. InE. coli the Ks value was about 260μM at greater than 5600μM and about 47μM at less than 5600μM substrate concentration.Uptake kinetics ofAeromonas sp. grown in a medium containing 2.7 mM glutamate (H-cell) and 0.11μM glutamate (L-cell) have been determined for the intact cells. H-cell had two distinct values of Km for glutamate assimilation and respiration, and L-cell had three distinct values of Km for glutamate assimilation and respiration: In H-cell Km of assimilation was 2.8×10(-7) M and 1.5×10(-4) M, and Km of respiration was 2.3×10(-7) M and 1.7×10(-4) M; in L-cell Km of assimilation was 7.4×10(-8) M, 8.3×10(-6) M, and 1.3×10(-4) M, and Km of respiration was 2.5×10(-7), 8.9×10(-6)M, and 1.7×10(-4) M. More than 60% of glutamate taken up by the H- and L-cells was respired when the substrate concentration was less than 10(-6) M, although at greater than 10(-6) M, 50% and 30% of glutamate was respired by H-cells and L-cells, respectively. These results suggest that the facultatively oligotrophic bacteria grow with high efficiency in environments with extremely low nutrient concentration, such as oligotrophic waters of lakes and ocean, as compared with in their growth in conditions of high nutrient concentraton, such as nutrient broth.