Genetic variation in some populations of the two edible oysters, Crassostrea rhizophorae and C. virginica, was studied using starch and polyacrylamide gel electrophoresis, coupled with histochemical stains. 19 enzymatic systems were analyzed, codified apparently by 30 loci in both species. The proportion of polymorphic loci was 16.7% in C. rhizophorae and 3.39% in C. virginica. The observed mean heterozygosity per locus and the mean effective number of alleles was 0.059 and 1.17 for C. rhizophorae and 0.021 and 1.03 for C. virginica.