Analysis of Western European populations of Asellus aquaticus uncovered 10 electrophoretic phenotypes of glucose phosphate isomerase (GPI) and 7 of phosphoglucomutase (PGM). Breeding studies indicate that the variation is controlled by codominant alleles at two autosomal loci. Genotype frequencies in the two sexes do not differ significantly, mating between genotypes is random, and no structural linkage is detectable between the two loci. PGM shows nongenetic, “secondary” banding, particularly in animals stored at −20°C prior to electrophoresis. This secondary banding confounds the identification of the genetic variation but can be controlled by the reducing agent 2-mercaptoethanol.