The impact of the invasive ctenophore Mnemiopsis leidyi on the zooplankton community of the Caspian Sea was quantified according to food consumption and other major physiological activities (i.e. respiration and reproduction), coupled with field data on population structure. The adverse effects of M. leidyi on the zooplankton community during the first years of the invasion were tremendous for the Caspian Sea compared to other regions affected by this ctenophore. The impact was highest in summer, due to high water temperatures and a population size structure in which juvenile ctenophores with mean lengths of 2 to 5 mm. accounted for most of the population. During winter/spring, these ctenophores could consume the available stock of zooplankton in 3 to 8 d, whereas in summer consumption took only 1 d. The computed critical ctenophore biomass that does not affect (decrease) the abundance of mesozooplankton in the Caspian Sea is about 4 g m(-3) (or 120 g m(-2), assuming most of the ctenophores occur in the upper 30 m layer). As is clear from the monitoring data, the M. leidyi biomass in summer in different regions of the Caspian Sea is far in excess of this value. Such a high abundance of ctenophores, if maintained, would constantly keep the non-gelatinous zooplankton biomass at very low levels, and, as a consequence, no recovery could be expected in the pelagic fishery.