Abundance of the exotic predatory cladoceran Cercopagis pengoi (Ostroumov, 1891) in the Gulf of Riga has increased during the first ten years of invasion (1992-2001) and peaked with an annual mean of 420 ± 109 (s.e.) ind. m-3 in 2001. As a long-term mean, Cercopagis population reached its highest abundance in the first week of August (mean 269 ind. m-3) and the seasonal population development followed the course of water temperature. Presence of Cercopagis in the plankton community tended to last longer over the years by shifting its population development to earlier in the year. The latter correlated significantly (r2=0.88, p<0.01) with a similar pattern of Eurytemora. After the invasion of Cercopagis, the annual mean abundance of Bosmina was significantly lower than during the pre-invasion time (t-test, p<0.01). Although populations of the other mesozooplankton taxa did not follow this pattern at the annual scale, seasonal population development of copepod nauplii and Acartia has changed, which can be caused by the invasion. The mean share of Cercopagis in the diet of most abundant planktivorous fish remained low (<7%) but the cladoceran constituted a substantial portion in the fish diet during warm months. Consumption of Cercopagis by herring, Clupea harengus membras, and smelt, Osmerus eperlanus, increased with increasing fish size. This was not evident in the case of sticklebacks, Gasterosteus aculeatus and Pungitiuspungitius. The study points out that the invasion of Cercopagis has caused changes in zooplankton dynamics. Therefore, dietary overlap with fish larvae and planktivorous fish may occur in the Gulf of Riga.