Abstract In this study, to reach the interactions between pollution and phytoplankton at the Golden Horn, the surface water samples collected at January-December 1995 period are investigated to aspect the phytoplankton qualitative and quantitatively. During the study period, 24 phytoplankton species (16 diatoms and 8 dinoflagellates ) were determined. At the entrance of the Golden Horn (Station 1) 22 species were determined but at the inner part (Station 5) only 8 species were determined in the whole of the year. The phytoplankton quantity (2 x 103 cells/L on an average) were in low concentrations in general and reached the highest value (146 x 106 cells/L) at July. During that increase Leptocylindrus minimus, a member of centric diatoms, was dominant with 95% ratio. Dinoflagellate species concentration reached the highest value (1679 cells/L) when Noctulica scintillans, a member of this group, was dominant with 93% ratio at November. According to the results of this study, the surface water phytoplankton community of the Golden Horn,. is quite poor at the point of species and individual quantity. That is explained; the phytoplankton species and individual quantity dt.'Crease from the entrance to inner side of the Golden Horn vice versa pollution increase at the same direction during this study period.