The distribution and abundance of diatoms and dinoflagellates which important members of marine phytoplankton were examined at three stations located from coastalmost area to offshore in the ÃÂ°skenderun Bay between 2004 and 2005. The abundance changes of diatoms and dinoflagellates with regard to sampling stations were investigated and the effects of environmental parameters were tried to determined. The diatom abundance ranged 0.25x103 and 18.9x103 cells l-1, its maximum were reached in June at the coastalmost station. The most abundant diatom species were Pseudonitzschia spp. and Proboscia alata. Dinoflagellate abundance ranged from 0.35x103 to 6.4x103 cells l-1 and their maximum were found in September at the offshore station. S. trochoidea and G.acuminatum were the most abundant dinoflagellate species. It was observed that diatoms were dominant throughout the study period. The results also indicated that diatoms were abundant at the nearcoastal area, while the dinoflagellates were more abundant at the offshore area.