Abstract: Annual variations in abundance, biomass, shape and size spectrum of picoplankton (coccoid cyanobacteria) were investigated to explain the blooming of diatom and dinoflagellates in the surface mixed layer in the Sinop Bay of the Southern Black Sea during the period between January 1998 and December 1998. At the end of this study, it was observed that the coccoid cyanobacteria were spherical in shape and 1.5-2.0 μm in diameter. It was determined that picoplankton were predominant in the phytoplankton, averaging 99.2% of the total phytoplankton abundance. The abundance and biomass of coccoid cyanobacteria dominated in the picoplankton, averaging 99.1 and 99.2% of the total picoplankton, respectively. The picoplankton biomass varied between 535 and 13477 mg m-3. Cell numbers of coccoid cyanobacteria were generally high and varied between 1.3x108 in April and 3.3x109cells L-1 in June. High cell numbers of 2.1x109and 2.3x109 cells L-1 were also observed in October and December, respectively. A monthly average of cell number and biomass was estimated as 9.5x108cells L-1 and 3961 mg m-3, respectively. Picoplankton abundances and biomass were not slightly correlated with physicochemical parameters. During the study period, the abundance (9.0x108 cells L-1) and biomass (3961 mg m-3) of coccoid cyanobacteria in the southern Black Sea was generally more higher than the data for different seasons and areas of other marine systems. Abundance and biomass in the surface mixed layer in research period, also revealed that picoplankton cells were more concentrated in the shallow waters than offshore waters.