The invasive ctenophore Mnemiopsis leidyi (Agassiz), which was transported from the Black Sea into the Caspian Sea at the end of the 1990s, has negatively affected the ecosystem of the Caspian Sea. Zooplankton abundance, biomass and species composition were evaluated on the Iranian coast of the Caspian Sea during 2001-2006. A total of 18 merozooplankton (13 species composed of larvae of benthic animals) and holozooplankton (four Copepoda and one Cladocera) species were identified. The total number of zooplankton species found here was 50% less than in a previous investigation performed in the same region in 1996 before the introduction of Mnemiopsis leidyi into the Caspian Sea. Cladocera species seemed to be highly affected by the invasion of Mnemiopsis leidyi; only one species, Podon polyphemoides, remained in the study area, whereas 24 Cladocera species were found in the study carried out in 1996. Whereas among the Copepoda Eurytemora minor, Eurytemora grimmi, Calanipeda aquae dulcis and Acartia tonsa that were abundant before the Mnemiopsis leidyi invasion, only A. tonsa (copepodites and adults) dominated the inshore and offshore waters after the invasion. The maximum in zooplankton abundance (22,088 +/- 24,840 ind.m(-3)) and biomass (64.1 +/- 56.8 mg.m(-3)) were recorded in December 2001 and August 2004, respectively. The annual mean zooplankton abundance during 2001-2006 was in the range of 3361-8940 ind.m(-3); this was two- to five-fold less than the zooplankton abundance in 1996. During 2001-2006, the highest abundance and biomass of Mnemiopsis leidyi were observed during summer-autumn months coincident with warm temperatures and generally when the abundance of other zooplankton organisms was low.