In order to determine the phytoplankton species in the Gulf of Iskenderun and the environmental factors that affect the distributions of the species, samples were collected horizontally with a plankton net at 9 stations in November 2005 and June 2006. As a result of the examination of the samples, 95 phytoplankton species belonging to 3 classes were identified. 12 species were new records for the Gulf of Iskenderun, one (Navicula transitans Cleve, 1883) of them was a new report for all the Turkish seas. Dinoflagellates were the majority group of the species composition (53.7%), followed by diatoms (45.3%). Most of the species identified in this study consist of phytoplanktonic algae reported in the Eastern Mediterranean earlier. These species were neritic, oceanic, temperate and subtropic. In this study, 18 species defined as potentially harmful algal species and three of them were also toxic. Primary hydrographic conditions, such as temperature (20.2-28.7 degrees C), salinity (31.0-39 ppt) and dissolved oxygen (7.21-8.60 mg l(-1)), were recorded on each sampling occasion. In addition, a check-list of the phytoplankton species recorded up to this date was given and totally 269 species were recorded from the Gulf of Iskenderun.