AbstractElectrophoretic variation of 13 presumptive enzymatic loci was studied in sticklebacks (Gasterosteus aculeatus) at 6 marine (Baltic) and 13 freshwater sites from Poland. Marine samples are significantly more variable (proportion of polymorphic loci, p # 0.38) than freshwater Samles (p # 0.12). Also the mean number of alleles per locus is significantly higher in the marine samples (n # 1.54) than in freshwater samples (n # 1.14). There is however no significant difference in values of mean heterozygosity between marine and freshwater sticklebacks. Some alleles are found in Baltic sticklebacks only, but all these occur in very low frequencies. Genetic distance between samples is low ranging from D = 0.000053 to 0.031410. However D is significant smaller between marine samples than between freshwater Samles. We suggest than this probaily is the result of larger genetic subdivision of freshwater populations. The low genetic divergence of marine and freshwater populations of the stickleback from poland might be expfained by the recent postglacial colonization of this area from a common refugium.