Aquatic Heteroptera of Great Rivers of the Ukrainian Steppe Zone and Seasonal Changes of Abundance and Biomass. Grandova, M. A. - Aquatic Heteroptera are one of the important components of water biocoenoses. During the study of the great rivers of the Ukrainian continental steppe zone 34 species were found, among them Sigara mayri (Fieber, 1860), a new species for Ukraine. Cymatia bonsdorffi i (C. R. Sahlberg, 1819) and Hydrometra stagnorum (Linnaeus, 1758) are recorded in the Ukrainian continental steppe zone for the first time. During the whole year the aquatic Heteroptera taxocenoses of all the concerned objects create the indivisible complex, which basis is formed by inhabitants of inundated reservoirs. Th e highest abundance is observed in the reed bed lakes in the middle of July, aft er the hatching of the second generation nymphs of the polyvoltine species, the maximum of biomass is observed in October in the reed bed lakes, where Nepomorpha gather for overwintering.