The Capercaillie is one of the most seriously endangered bird species in Poland. It currently numbers around 650 individuals that live in four isolated populations (Lower Silesian Forest, Janów Lubelski Forest, Carpathians, Augustów Forest). This study investigated genetic variability based on the polymorphism of six microsatellite loci in the surviving Polish populations of the Capercaillie and compares the results with the analogous variability in two large, contiguous populations in Russia. The following parameters were estimated: mean number of alleles per locus, allelic richness, mean effective number of alleles per locus, heterozygosity in each of the populations investigated. Differentiation between pairs of populations was assessed using F_ST. The results show that despite some inevitable reduction in genetic variability, most of the Polish populations retained a substantial level of microsatellite polymorphism. Only in the population from Janów Lubelski Forest was there a significant reduction in variability, probably due to long isolation and the recent decline. That this population has long been isolated was also confirmed by the pronounced genetic differentiation from the other Polish populations. The Carpathian population of the Capercaillie was found to be genetically structured, and in the Lower Silesian Forest population heterozygosity was low, possibly as a result of the lek mating system and also the dramatic reduction in numbers.