In this study, spatial and vertical distributions of the invasive ctenophore Mnemiopsis leidyi in the Caspian Sea were evaluated by using data collected at 41 stations during the August 2001 cruise. A comparison of data from different depths revealed that M. leidyi were generally confined to surface waters. The maximum size of the ctenophore was only 41-45 mm, and the bulk of individuals (85.5%) were < 10 mm in length. The average and maximum biomasses of M. leidyi were calculated as 120 and 351 g wet weight m(-2), respectively. Whilst highest biomasses were observed in the western and central Middle Caspian Sea, hot spot areas of reproduction were present along the coasts of the western Caspian Sea, with abundance values of up to 2285 ind. m(-2). The impact of such high densities of M. leidyi is expected to be significant for the pelagic ecosystem of the Caspian Sea.