In order to understand the temporal distribution of pico- and nanoplankton and facters controlling its distribution at a station in Okkye Bay of Masan Bay located in the southern part of Korea, this study was conducted on two weeks interval from April 2005 to April 2006, and several abiotic and biotic factors were measured. During the study, picoplankton consisted of picoflagellates, cyanobacteria and heterotrophic bacteria, and nanoplankton consisted of nanoflagellates excluding dinoflagellates. The concentration of chlorophyll-a(chl-a) was a mean of 4.33㎍/L, and the nanoplanktonic(＜20㎛) chl-a size fraction was a mean of 39.5% and significantly correlated with water temperature. The abundances of cyanobacteria and photosynthetic flagellates(PF) were means of 24.4×10³cells/mL and 2.87×10³cells/mL, respectively. The contribution of picoflagellates to the PF abundance varied among the sampling occasions and was a mean of 29%, but to the PF carbon biomass was 2.6% only. The PF abundance had significant relationships with water temperature, and silicate and TIN concentrations, suggesting that the PF abundance seemed to be primarily bottom-up regulated. The abundance of heterotrophic bacteria was a mean of 3.18×10⁶cells/mL and unlike other ecosystems it did not have relationships with chl-a and heterotrophic flagellates(HF), suggesting that bacterial abundance did not seem to be bottom-up or top-down regulated. HF mostly consisted of cells less than 5㎛ and its abundance was a mean of 2.71×10³cells/mL. Of the HF abundance, picoflagellates occupied about 31%, and occupied about 9% of the HF carbon biomass. HF grazing activity on heterotrophic bacteria was relatively low and removed about 10% of bacterial abundance, suggesting that HF might not be major consumers of bacteria and there seems to be other consumers in Okkye Bay. These results suggest that Okkye Bay may have a unique microbial ecosystem.