This study was carried out to determine the diurnal distributions of phytoplankton density, bio-volume, and chlorophyll a in relation to nutrients and CTD measurements in surface waters of the coastal area in the Dardanelles during high density of phytoplankton, especially high diatom production (between 3 July and 4 August 2002). Ninety nine surface samples were collected for nutrient, chlorophyll a, and phytoplankton in 3 different time periods of the day (08:00, 13:00 and 18:00 hours) in the sampling period. Based on the sample analyses, average temperature, salinity, pH, DO, TDS, TSS, NO2-+NO3-, PO4-3, SiO4, chlorophyll a, N:P, and Si:P were calculated as 25 degrees C, 23.3 ppt, 8.42, 9.26 mg L-1, 23.9 gL(-1) 36.1 mg L-1, 1.90 mu M, 0.24 1 mu M, 3.6 mu M, 1.70 mu g L-1 12.5, and 25, respectively. Total phytoplankton density and bio-volume varied from 2.86 x 10(5) to 1.5 x 10(7) cells L-1 and from 5.98 x 10(8) to 8.81 x 10(10) mu m(3)L(-1), respectively. Contribution rate of Bacillariophyceae (66.5%) to total phytoplankton bio-volume was higher than that of Dinophyceae (31.0%). Phytoplankton community structure was observed to be controlled by 6-7 species. Other species can be considered as insignificant species that do not cause important fluctuations in the phytoplankton density and bio-volume. It has been reported that there were 9-11 population growth slopes for different species at different times and densities. Therefore, life cycles of phytoplankton species were completed between 3 and 4 days. Diurnal variations of biological and physicochemical parameters in the time interval between 0800 and 18:00 were generally much higher than daily variations between 3 July and 4 August 2002 due to the 2 counter flows system of the Dardanelles and domestic inputs of canakkale city.