The selectivity of amino acid assimilation by marine bacteria was examined using seven kinds of14C-amino acids and the acid hydrolysate of14C-labelled proteins. It was found that the net assimilation and respiration by marine bacteria followed MICHAELIS-MENTEN kinetics for all of amino acids used in our experiments. Maximum velocities of amino acids were 0.01 to 0.19Μg carbon/hour per 2×107 cells for net assimilation and less than 0.18Μg carbon/hour per 2×107 cells for respiration at 20‡C. The velocity of gross assimilation was found with the following order: phenylalanine>valine, glutamic acid>serine, arginine>tryptophan>glycine. The assimilation velocities of amino acids in these laboratory works showed almost the same order as those in field experiments. The assimilation velocity of an amino acid was influenced by coexisting another amino acids or glucose. The assimilation velocity in lower substrate range of amino acids was directly proportional to the number of bacterial cells in the range from 6×102 to 3×104 cells per ml. No linear relation between the assimilation velocity of amino acids and reciprocal of absolute temperature was found, but a marked bending was observed at 15‡ to 20‡C. The velocity at the optimum temperature was three to six times of that at 5‡C.