By the use of a chemostat the growth kinetics of two different bacterial populations which showed different responses to dissolved organic substances in the natural sea water was studied. The kinetics constants for the growth of Z1-bacteria, grown on a nutrient-rich medium, were: Ks=3.0mgC/l and βm=0.19h-1, and those for the growth of Z/20-bacteria, grown on nutrient-poor medium, were: Ks=1.2mgC/l and βm=0.15-1. Based on these data the relationship between the structure of bacterial community in the sea and the concentration of organic substances in sea water is discussed.