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The research both of the quantity and size structure of the invader Mnemiopsis leidyi (A. Agassiz 1865), as well as the mesozooplankton abundance, species diversity, the food spectrum, trophic relations, and predatory impact by M. leidyi on prey populations in the western and eastern sectors of the open Black Sea was carried out in July - August 2017. M. leidyi biomass varied from 92 to 250 g m during the period of maximum seasonal development. The food spectrum of M. leidyi was dominated by copepods Acartia spp. and Calanus euxinus (Hulsemann 1984). A clear spatial difference in the quantity of M. leidyi and zooplankton, the intensity of food consumption and predatory impact was observed. In the western offshore part of the sea, the high food ingestion rate by M. leidyi (the mean specific daily ration was 45.50±8.31% С) and intensive predatory impact on zooplankton (up to ~50% of Acartia sp. biomass) resulted in the strong decrease in mesozooplankton biomass. In contrast, in the eastern sector M. leidyi predation had no effect on zooplankton community maintaining daily grazing of ~6% Acartia spp. biomass.