Thirty two populations of Dreissena polymorpha (Pall.) from Poland were electrophoretically studied with respect to seven enzymatic loci. The zebra mussel was found to be genetically highly variable, as indicated by high polymorphism of particular loci and wide individual genotypic variation. The parameters of genetic variation of D. polymorpha from Poland were: 100% polymorphic loci, mean number of alleles per locus 4.7, mean number of genotypes per locus 10.3, mean expected heterozygosity per locus 0.473. The mean percent- age of polymorphic loci per population was 89.3, mean number of alleles per locus per population 3.5, mean number of alleles per polymorphic locus per population 3.7, mean expected heterozygosity per locus per popu- lation 0.447, mean number of genotypes per population 5.3. Colonies of D. polymorpha proved to be genetically much differentiated: the mean percentage of polymorphic loci per colony was 86.2, number of alleles per locus 2.62, genotypic differentiation 95%. The genetic similarity between the populations was high which indicates a large uniformity of the species in Poland. The genetic similarity between the populations was 0.828–0.999, the genetic distance 0.001–0.189. No genetic differences were found between populations from the regions of Pomerania, Mazurian Lakeland and Konin. The genetic similarity between the three groups of populations ranged from 0.975 to 0.986, the genetic distance from 0.014 to 0.025. The Polish populations of the zebra mussel showed a slightly higher level of genetic variation compared to W European and N American populations. Colonisation is not accompanied by a decrease in the gene pool resulting from genetic drift; the species seems to expand its range using all its genetic potential.