In order to estimate the genetic variability and differentiation in common squid, eleven isozymic loci, coded for nine enzymes detected by starch gel electrophoresis, were scored from nine spawning cohorts in four localities. The expected average heterozygosity ranged from 0.00019 (between II-S$_2$ and Na-W) to 0.00814 (Between Bu-S and Na-W) in nine different spawning cohorts in four localities. A dendrogram, based on genetic distance mentioned, illustrated that nine different spawning cohorts were divided into three groups, similar to the result estimated by their ecological characterizations. From these results, we estimate that the common squid distributed throughout Korean waters will maintain this gene exchange. It is postulated that either the summer or the autumn spawning cohort has developed a local population that is isolated by hydrographic factors.