We investigated the seasonal abundance of the dinoflagellate Heterocapsa triquetra (Ehrenberg) F. Stein, as well as the relevant in situ environmental factors, in brackish Lake Shihwa, Korea. We also examined the growth rates and morphological characteristics of the species in laboratory cultures. In the field, the population densities of H. triquetra remained at low levels from late spring to early summer, and then completely disappeared from August to November 2007. Interestingly, a dense bloom of H. triquetra appeared below the ice surface on 17 January 2008; identities of the cells were confirmed by rDNA sequence comparisons. The second peak reached a density of 672 × 103 cells L−1 on 28 March 2008, at a water temperature of 9.1°C. Laboratory experiments showed that growth rates of H. triquetra increased with incremental temperature increases within the range of 10 and 20°C. The highest growth rate reached by H. triquetra was 0.62 d−1 at 20°C with a salinity of 30. Above 25°C, the dinoflagellate was unable to grow between salinities of 10 and 15, and reached only relatively low growth rates (<0.12 d−1) under other salinity conditions. However, under continuous cultures at 5°C and 8°C, H. triquetra cells retained its growth capability for more than 12 days, implying that H. triquetra can survive and grows even at very low temperatures. The equivalent spherical diameter (ESD) of H. triquetra did not change markedly between 10 and 25°C, but the equivalent spherical diameter was significantly different at 5°C. The cell volume buildup of H. triquetra at low temperatures is one of the important survival strategies to overcome the harsh environmental conditions. These characteristics make H. triquetra a consistently dominant dinoflagellate in Lake Shihwa during the cold winter season.