The research was carried out in 1994-2000 from spring till autumn, in a mesotrophic Lake Piaseczno situated in the Łęczyńsko-Włodawskie Lakeland. The Shannon-Wiener species diversity index H’ was determined for the studied lake zones, and fauna similarity was calculated on the basis of the homogeneity index [15]. In the lake, 37 species of Gastrotricha were found. In bottom sediments of littoral with sublittoral 22 species were abundant, and on submerged vegetation 20 species. In the psammon zone of the lake 11 species were found. The lowest value of H’ was obtained for bottom sediments in the profundal zone of the lake (1.49). The most separate Gastrotricha fauna is abundant in bottom sediments of lake profundal, for which similarity with the rest of the lake zones amounts to 9.9-27.3%. In the lake eudominants (D > 10,0%) there were 10 species of Gastrotricha. Total dominance of eudominants in individual lake zones varied from 34.1% (shore) to 82.1% in lake profundal. The research carried out allows the completion of the autecological characteristics of the following species: Ch. macrochaetus and Ch. disiunctus which were previously found mainly on peat bogs and which appeared to be common eurytopic species. But Ch. similis and L. squamata are mainly lacustrine species.