A study on the abundance and composition of some groups of phytoplankton (diatoms, dinoflagellates and silicoflagellates) was carried out in the marine areas of the Datca and Bozburun Peninsulas between 2002 and 2004. Simultaneously, measured physical (salinity, temperature, secchi disc) and chemical parameters (nutrients, chlorophyll a, dissolved oxygen) were assessed together with phytoplankton data. Seawater and plankton net samples were taken from 63 stations during 6 sampling periods. A total of 132 taxa (genus, species and infraspecies level) belonging to 3 taxonomic classes were reported and a checklist of phytoplankton was prepared for this study area. Average nutrient values in surface water ranged from 0.01 to 1.19 mu M for NO3+NO2-N, from 0.01 to 0.69 mu M for PO4-P and from 0.50 to 5.31 mu M for SiO2-Si and chl-a values were between 0.19 and 0.68 mu g l(-1) throughout the study area. The highest number of phytoplankton cells reached 5400 cells l(-1) and dinoflagellate Prorocentrum micans reached 1500 cells l(-1) while diatom Thalassionema nitzschioides reached 700 cells l(-1). Dinoflagellates showed a more homogeneous distribution in a wider area than diatoms. Dinoflagellate abundance increased in areas close to the fish farms due to the amount of nutrients originating from the farms. Spatial changes in phytoplankton composition observed in this marine area revealed that phytoplankton is very sensitive to ecosystem changes. The study area could generally be defined as oligotrophic in terms of trophic status, depending on the nutrient and chl-a concentrations. Moreover, very low cell abundance and the high number of species observed in this area also reflect the typical characteristics of oligotrophic waters.