Changes in abundance and biomass of Synechococcus were studied during five cruises carried out between November 2004 and September 2005 in the Iskenderun Bay, northeastern Mediterranean. In addition, spatial and temporal variations in physico-chemical factors and Chl a were measured. Abundance ranged from 0.2 x 10(4) to 23.09 x 10(4) cells ml(-1), whereas the biomass ranged from 0.19 (1) to 23.01 mu gC l(-1) in the bay. Synechococcus was found most numerous during September 2005 in the shallower part of the bay while the population was observed least during November 2004 and January 2005. The contribution of < 3 mu m size fraction to total Chl a ranged between 48%, and 74%, being low during the cold period and higher during the warm period. The observed spatial and temporal fluctuations in abundance seemed to be closely related to environmental conditions in the area. Small-sized phytoplankton dominated the bulk of biomass, at least for the summer period.