Zooplankton was studied seasonally between 2009 and 2011 in Iskenderun Bay in order to determine temporal variations in abundance, biomass, and size structure and their relationships with environmental factors. Zooplankton sampling was performed vertically using a WP-2 net (200-mu m). A total of 30 zooplankton taxonomic groups were identified. Cladocera was the dominant group, and together with Copepoda and Appendicularia constituted approximately 90% of zooplankton. In respect to abundance and biomass values, 2 peaks were observed in 2009, the first in spring and the second in autumn. However, a second peak was observed in summer in both 2010 and 2011, due to increases in cladoceran abundance. During all seasons and years of the study period, small-sized organisms (200-500 mu m) constituted the majority of the zooplankton. Zooplankton abundance was influenced by environmental factors. The relationship between the abundance of zooplankton groups and environmental parameters varied according to the groups. In conclusion, clear seasonal variation and slight differences among years were observed in terms of the examined properties of the zooplankton groups. Abiotic environmental variables and availability of food were considered to particularly affect this variability.