Seasonal variations of zooplankton communities were studied in the southern Caspian Sea for 4 successive seasons and 8 transects with 5 stations from 5 to 100 m depths which sampled during 2009. The zooplankton population constituted of 73.33% copepods, 24.21% rotifers, 2.23% cladocerans and 0.23% protozoans. Copepods were dominant at all stations with a density between 1456±531 ind. m-3 (Transect 8) to 4524±1215 ind. m-3 (transect 2). The predominant species of copepods was Acartia tonsa constituting 99.50% of all copepod populations. They were most abundant during warm months of summer in the upper layers (surface to 20 m depth) while rotifers replaced them during cold season. The maximum density of zooplankton was observed in the west decreasing towards the eastern parts of the sea.