The main objective of the study was to present an analysis of the Hydrachnidia species composition in the highly varied Lake Świdwie ecosystem at the macro-and mesohabitat level. 73 species were recorded. Species associated with small water bodies were most abundant, together with lake species. The most individuals and species were caught in the Phragmites-Close mesohabitat. The fewest individuals were caught in the Phragmites-Typha mesohabitat, and the fewest species in the Chara mesohabitat. Three groups of species associated with particular mesohabitats were recorded: 1) species associated with the Phragmites-Close mesohabitat, 2) species associated with the Thelypteris palustris-Phragmites australis and Phragmites-Typha mesohabitats; 3) species associated with the Chara and Ceratophyllum-Myriophyllum mesohabitats. The high similarity between the fauna of the two lake basins and the marked faunal distinctiveness of the Hydrachnidia assemblages in the small pools on the margins of Lake Świdwie indicate that it was the character of the water bodies that determined the composition of the Hydrachnidia assemblages in each of the macrohabitats of the Świdwie reserve, whereas the influence of their surroundings was negligible. The results obtained confirm our hypothesis that in the highly varied system of water bodies and habitats one can expect substantial diversity of Hydrachnidia.