Research relating to the subject of this paper was conducted on the Tywa River between September 1996 and July 1998. In the samples, invertebrates were found belonging to 46 species, 26 genera, and two subfamilies. In the 1996 season, the highest abundance was observed among Tanypodinae (larvae) at 10,878 ind. m-2; among Chironominae (larvae) it was 9,965 ind. m-2; and among Tubifex tubifex it was 2,605 ind. m-2. The maximum total density of macrozoobenthic forms in the Tywa River was at station no. 1, where it amounted to 23,953 ind. m-2. The quantitative structure in the 1997 season was lower than in the 1996 season, and the highest abundance was for Simulium sp. (larvae) at 5,196 ind. m-2; for, Tanypodinae it was 3,360 ind. m-2; and for Dreissena polymorpha it was 4,721 ind. m-2; and for Theodoxus fluviatilis — it was 4,114 ind. m-2. In this season, the biodiversity of the macrozoobenthic community was the highest of all seasons. In 1998, the qualitative structure of the macrozoobenthic community was low: Hirudinea disappeared, and the quality of the community was almost four times lower than in 1996. The river's fast current, its diverse substratum, its very good oxygenation, and moderate concentrations of organic matter created conditions appropriate for species that differ in their requirements.
