Electrophoretic variants at 28 genetic loci were analyzed in subspecies of Peromyscus maniculatus endemic to the Channel Islands of the California coast. The genetic variability within insular populations was calculated. These deer mice have relatively high levels of polymorphism for insular populations. The mean heterozygosity per individual varies in the populations, being much higher on one of the islands than the others. Nei's measure of genetic distance between groups compared on the basis of electrophoretic variants was used. His estimate of time of divergence of these groups, based on genetic distance, is applicable particularly to closely related groups. The length of time each island population has been isolated from the others was calculated and found to be consistent with the periods of isolation estimated on the basis of geological data.