In the present study, seasonality and composition of phytoplankton and zooplankton were comparatively studied at two stations (one inshore and one offshore) off Sinop, in southern Black Sea during January-May 1999 and March-October 2000. A total 76 genera including 149 species and varieties were identified, of which 46% were dinoflagellates and 34% were diatoms in phytoplankton. Abundance of the genera Emiliana (in January-May 1999), Pseudosolenia and Ceratium (in March-October 2000) were higher than that of the other genera in almost every sampling period. Increased dinoflagellate to diatom ratio reflects more fertile conditions in the southern Black Sea. Abundance and biomass of the total fodder zooplankton the highest values were recorded in February 1999; March 2000. Cladocera and Appendicularia abundance and biomass values were higher in 1999 than 2000. Noctiluca scintillans was the most abundant of zooplankton group with a share of 43% (maximum: 416,925 ind.m-2 in May) in 1999. The second abundant group was Copepoda with 42% (172,981 ind.m-2 in February). Copepods with 78% of abundance and 73% of biomass (maximum: 372,330 ind.m-2 and 15,253 mg.m-2 in March) were the dominant group in 2000. They were followed by the dinoflagellate Noctiluca scintillans with 4.8% (maximum: 24,500 ind.m-2 in June).