This thesis presents the results of an ecological study of cladocerans in Eastern Adriatic waters under significant freshwater influence. Zooplankton samples were collected at five stations in the estuaries of the Krka and Zrmanja rivers and in the Neretva Delta, and at one station in the open North Adriatic during all seasons (winter, spring, summer, and autumn). The total abundance of cladocerans varied in a wide range from 0.1 ind. m-3 (Zrmanja River estuary) to 1346 ind. m-3 in the northern Adriatic. The greatest variation in abundance was observed in April and the least in November. Marine species Penilia avirostris, Pseudevadne tergestina, Evadne spinifera, Evadne nordmanni, Podon intermedius and Pleopis polyphemoides were found in the samples. The occurrence of freshwater cladocerans Bosmina longirostris and Daphnia sp. was noted in the winter/spring period when they were brought into the marine environment with abundant river water after the rainy season and snowmelt in the hinterland. Species P. intermedius, P. polyphemoides, and E. nordmani were predominantly present in the winter/spring, while the remaining species (P. tergestina, E. spinifera, and P. avirostris) were more abundant in the warmer season. The analysis of the population structure of each species by sex and stage showed the quantitative dominance of parthenogenetic females; non-reproductive females were found only in the population of P. avirostris at the end of summer, males of P. tergestina and E. nordmanni in April and September, and gamogenetic females only in E. nordmanni. No major differences in dimensions and morphology of taxonomically relevant body parts were found in Adriatic individuals compared to previously published descriptions.