Phytoplankton biomass. abundance, diversity and their relations with physico-chemical properties of water were determined in the northwestern part of the iskenderun Bay, northeastern Mediterranean coast of Turkey, Nine stations were sampled monthly from July 2000 to July 2001. Phytoplankton biomass, in terms of chlorophyll a values ranged from 0.05 to 2,7 mug.L-1 during the study period. Phytoplankton abundance was lowest in summer (2.64x10(2) cells L-1) and highest in spring (127.25 x10(2) cells L-1). Diatoms were found to be present throughout the year. They were the dominant group in terms of both number of species and abundance (66%). Dinoflagellates constituted 31% of total abundance. Diversity decreased when phytoplankton abundance increased showing an inverse relationship. The mean values for species diversity and evenness indices during the study period were 2.96 +/- 0.45 and 0.80 +/- 0.12, respectively.