The monthly changes in chlorophyll a, phytoplankton abundance and nutrient concentrations at two stations, one at the inshore and the other at the deep waters of the northern part of Iskenderun Bay, were investigated between 1994 - 1995. The vertical distribution of nutrients and phytoplankton biomass were also studied at the deep station. The concentrations of NO3+NO2-N, PO4-P and SiO4-Si of surface water at both stations were 0.31 - 1.63 mug-at . l(-1), 0.08 - 0.60 mug-at . l(-1) and 0.50 - 2.7 mug-at . l(-1), respectively. The highest concentrations were measured at the inshore station and clear differences were found between the inshore and deep-water stations. Chlorophyll a concentrations ranged from 0.17 to 2.78 mug . l(-1) and the highest value was measured in March. At the inshore station, which was affected by land run-off, phytoplankton abundance reached the highest value (21,308 cells . l(-1)) in October 1995, with a marked dominance of Pseudonitzschia pungens (20,200 cells . l(-1)). The nutrient and chlorophyll a concentrations at the inshore station were higher than those at the deep station. One reason for this is the land-based nutrient input into the coastal area here. In spite of these effects, the bay is not eutrophicated because of circulation events in the northeastern Mediterranean.