Ceratium species are not a common component of freshwater phytoplankton in South America. However, these dinoflagellates have often been observed in many water bodies over the past two and a half decades. We investigated Ceratium furcoides' abundance and morphological variation during its initial phase of colonization (2012)(2013) in two subtropical reservoirs in southern Brazil in order to explore which environmental factors were related to the occurrence, persistence and bloom formation of this dinoflagellate in those environments. Biomass of C. furcoides showed a strong seasonal pattern, in which warm seasons led to an increase in population density, resulting in cell-size reduction, while in the cold seasons cells increased in volume. Maximum densities over 2,500 cells ml -1 were observed in spring-summer periods in both reservoirs. C. furcoides' abundance in the studied reservoirs was associated, primarily, with a combination of optimal conditions of temperature, organic matter, and pH, and secondarily, with nutrient availability. The possible factors for the successful colonization performed by C. furcoides across distinct Brazilian waterbodies include good swimming performance, low herbivory pressure, and ability to form dense blooms, as strategies that allow maintenance of populations and effective dispersal.