Nei's (1972) genetic distance were estimated among 21 populations of the common freshwater shrimp Paratya compressa improvisa, living in the northern part of Japan. The estimates were based on examination of 18 enzyme loci by starch-gel electrophoresis. These shrimp populations were genetically classified into three geographic groups on the basis of predominating alleles typical for two loci Fdp-1 and Fdp-2. The “Kanto group” consisted of individuals having Fdp-la and Fdp-2c, the “Japan Sea group” having Fdp-1b and Fdp-2a, and the “Pacific Ocean group” having Fdp-lb and Fdp-2c. Nei's genetic distance among the three different geographic groups, varied from 0.0572 to 0.2136. Average genetic distances were 0.1001 between Pacific Ocean and Japan Sea groups, 0.0851 between Pacific Ocean and Kanto groups, and 0.1990 between Japan Sea and Kanto groups. These values of genetic distance among the three groups are comparable with those among different subspecies.