The taxonomic composition, biotopic distribution, and interannual dynamics of the population of water mites in Lake Kuchak (Western Siberia) have been studied. The occurrence has been analyzed. The dominant species in different biotops have been revealed. Forty-three species of water mites from 12 families are recorded. Acarofauna is primarily composed of eurybiontic species typical for stagnant waterbodies. The highest species diversity and a large number of mites are the most usual characteristics of shallow water areas with well-developed vegetation. The age structure has been shown to influence the distribution. It has been revealed that mature specimens seem to prefer life in shallow water, while specimens at the larval and nymphal stages tend to live at greater depths.