Chlorophyll a concentrations of size-fractionated samples were measured in the NE Mediterranean Sea (Iskenderun Bay). Data were collected during four cruises performed in January, April, July and November 2002 in Iskenderun Bay. The spatial, temporal and vertical variations of size-fractionated chlorophyll a and taxonomic composition of phytoplankton were investigated. Some hydrographical factors were measured and inorganic nutrient concentrations determined. The total chlorophyll a concentration and those of the three size classes microplankton (>20 mu m), nanoplankton (5-20 mu m) and picoplankton+ ultraplankton (<5 mu m) were determined. The relative importance of picoplankton + ultraplankton fraction was highest during the research period. Its averaged contribution ranged from 43.3% to 52.5% of the total chlorophyll a. The microplankton fraction was the second group in terms of percentage abundance, and accounted for 25.5%-34.4% of total chlorophyll a, and the level of nanoplankton fraction was the lowest. contributing 19-22.2 % of total chlorophyll a. Among the hydrographic factors, temperature was correlated with all size fractions as well as total chlorophyll a. The taxonomic investigation of the phytoplankton revealed that the diatoms were abundant. In addition to phytoplankton abundance and nutrient levels, the important contribution of picoplankton + ultra plankton fraction to phytoplankton biomass in terms of chlorophyll a concentrations in the present study reflects the oligotrophic character of this region.