We investigated the abundance and biomass of dinoflagellates and factors controlling their abundance in marine planktonic ecosystems in Korean coastal waters. The abundance of photosynthetic (PDNF) and heterotrophic dinoflagellates (HDNF) was in the range of 0.7${ imes}$10$^2$ cells/1-14.0${ imes}$10$^6$ cells/1 and in the range of 3.0${ imes}$10$^2$ cells/1-6.47${ imes}$10$^5$ cells/I, respectively. Their biomass was 0.5${ imes}$10$^{-1}$-2.56${ imes}$10$^4$ ${mu}gC/I$ and 2.0${ imes}$10$^{-1}$-1.5${ imes}$10$^{2}$ ${mu}gC/I$, respectively. In order to find factors controlling their abundance, stepwise regression and best subsets regression analyses were used. We found that during the summer the most important factors controlling PDNF abundance are DO, P, N and S (abiotic factors), and for HDNF, the abundance of zooplankton, ciliates and HF (biotic factors), and that high turbidity may effect the distribution of dinoflagellate species.