The Black Grouse Tetrao tetrix populations in Europe have become fragmented and reduced in numbers. These processes affected genetic diversity of the species, altering patterns of gene flow and genetic structure. In Poland, the Black Grouse is one of the most endangered bird species, however very little is known about diversity of this native population, bordering on to the area of Europe in which the species maintains a continuous range. To complete the knowledge of genetic structure of European populations, we analysed microsatellite polymorphism in 97 native Polish birds, along with 39 introduced individuals, originating in Belarus and at a breeding centre in Poland, regarded as representing the gene pool from the north-eastern part of the country. The results confirmed that isolation of populations and recent decreases in their sizes have reduced genetic diversity in Poland's populations of the Black Grouse. The results also indicated the presence of two genetic groups in Poland, involving birds of the north-eastern part of the country and the Carpathians Foothills in a first group, and the population from the Sudetes (Izerskie and Karkonosze Mountains) in a second. We suggest that the conservation effort should treat these two groups as independent units. We also analysed a fragment of the mitochondrial Control Region (CRmtDNA). The comparisons of sequences obtained with data from other Eurasian populations indicate that populations of the Black Grouse in Poland should be included within a large Conservation Unit — the northern tetrix — which comprises birds from Eastern and Northern Europe.