Summer phytoplankton species were investigated in the Sea of Marmara between 1993-1995. The samples were collected in the months of June-August from the subsurface (0.5 m) with a 55 mum net at 52 stations. A total of 102 phytoplankton species were identified. Of these, 19 species two of them at the generic level, are new to the region. Diatoms and dinoflagellates were present throughout the sampling period and diatoms represented the majority of the population (46.08%), followed by dinoflagellates (44.12%). The others formed only 9.8% of the whole population. The genera Protoperidinium, Ceratium and Chaetoceros were dominant in terms of diversity. Primary hydrographical observations like salinity, temperature and dissolved oxygen were recorded on each sampling occasion. The species number of phytoplankton was negatively correlated to salinity (r = -0.33, p<0.05). The other parameters did not appear to play any role with the species number. Salinity was positively correlated to temperature (r = 0.64, p<0.05) and negatively to dissolved oxygen (r = -0.42, p<0.05). Furthermore, dissolved oxygen was negatively correlated to temperature (r = -0.30, p<0.05).