The seasonal dynamics of phytoplankton communities were investigated in the coastal zone of Iskenderun Bay, northeastern Mediterranean Sea with regard to some environmental factors during a one-year period. The samples were collected at monthly intervals from three depths in iskenderun Bay, from April 2003 to March 2004. A total of 168 phytoplankton taxa were identified. Phytoplankton abundance showed characteristic oligotrophic water values and ranged from 0.3 to 430 x 103 cells L-1. Diatoms were dominant throughout the year and represented the majority of phytoplankton. Diversity attained the lowest value in June due to bloom of a diatom, Leptocylindricus minimus Gran. A shift from small, chain-forming diatoms to large species was observed during late summer and autumn. Dinoflagellates were low in abundance and did not show regular variations during the study period. Small peaks were observed, which was caused by sporadic proliferations of some coastal species. The dominance of diatoms over dinoflagellates during the one-year period seems to be related to nutrient availability from land runoff and strong mixing of water. Active physical processes due to the hydrographically dynamic structure of the area were the main factors determining the community's structure and succession.