The toxic dinoflagellate Gymnodinium catenatum isolated from the southern coast of Korea was described under light and scanning electron microscopy, and its large subunit (LSU) rDNA was sequenced. In addition, the effects of temperature and salinity on its growth were investigated. The cells of G. catenatum, as viewed under the electronic microscope, were green-brown color, 38.1-77.4 μm in length and 26.1-40.8 μm in width. The epicone was conical, while the hypocone was trapezoidal. The nucleus was located at the central part of the cell. The apical groove was horseshoe-shaped and small pores were irregularly distributed on the cell surface. Molecular phylogeny based on LSU rDNA gene sequences showed that the Korean G. catenatum and previously reported species formed a monophyletic clade within Gymnodinium sensu stricto clade. The maximum growth rate of 0.37 day-1, was obtained at 25°C and 35 psu, and the maximum cell density of 1,073 cells mL-1, was observed at 20°C and 25 psu. However, G. catenatum did not grow at temperature <15°C and <30°C. These results suggest that environmental conditions of summer and autumn in the southern coast of Korea may be favorable for the growth of G. catenatum.