The aim of this study is to characterize the seasonal variation of zooplankton and the impact of some ecological variables on zooplankton distribution. The study was conducted seasonally in the Gulf of Erdek between November 2006 and August 2008. Zooplankton samples were collected from 3 sampling stations using WP-2 zooplankton net (200 mu m mesh size). During the study, 2 holoplankton, 7 meroplankton groups, 12 Copepoda, 3 Cladocera, 1 Ctenophora and 1 Cnidaria species were found in the study area. Acartia clausi (401 ind/m(3) in May 2008), Paracalanus parvus (139 ind/m(3) in February 2007) which belong to Copepoda, Penilia avirostris (610 ind/m(3) in August 2008) which belong to Cladocera and Liriope tetraphylla (270 ind/m(3) in August 2007) which belong to Cnidaria were the important species of this study since they are abundant in total zooplankton identified. In addition, temperature (6.5-25.5 degrees C), salinity (22.4-38.8 ppt), dissolved oxygen (3.78-14.96 mg L-1), chlorophyll-a (0.1-2.83 mu g L-1), NO2+NO3-N (0.11-4.93 mu g-at N L-1), PO4-P (0.09-2.11 mu g-at P L-1) and SiO4-Si (0.28-21.62 mu g-at Si L-1) of seawater were recorded on each sampling occasion.