Five populations of the zebra mussel Dreissena polymorpha, from small, isolated, post-glacial water bodies and one reservoir formed in the 50s (all in the Western Pomerania) have been examined. Isoenzyme electrophoresis in starch gel was used in order to estimate their variation and genetic structure. The studied populations are widely variable genetically, and their genotypes are very diverse, in spite of their isolated character. The analysis revealed 86% polymorphic loci, 2.3–3.7 alleles per locus and 2.5–4.2 per polymorphic locus, 2.4–5.4 genotypes per locus and the expected heterozygosity of 0.338–0.487. The genetic similarity between the populations ranged from 0.828 to 0.949 and was somewhat lower, compared to other populations of the species. The level of genetic variability of D. polymorpha in the isolated populations was comparable to that found in large populations from the Western Pomerania and with the founder populations from the Great Lakes of North America. Colonisation by D. polymorpha is not accompanied by impoverishment of gene pool resulting from founder effect. The species seems to be expanding massively, using all of its genetic potential.