SummaryExtracts of the digestive diverticula of more than 300 individuals from four geographically separated populations of the European oyster,Ostrea edulisL. have been examined by electrophoresis for esterase and phosphoglucose isomerase polymorphisms. Three regions of esterase variability have been detected in the electrophoretograms. It is inferred that the variants in two of these regions are governed by two co‐dominant alleles at each of two loci. Two phenotypes of phosphoglucose isomerase, which is inferred to be a dimeric enzyme encoded by two alleles at a single locus, have been observed. Allele frequencies and phenotype distributions are compared in the four populations. It is concluded that the populations differ genetically at the loci investigated, the magnitude of the differences being generally greatest between different ‘physiological rac