Frequency of phytoplankton community in reservoirs depends on several physical, chemical and biological factors, which in turn are subjected to rainfall regime. This work presents temporal variations of limnological parameters, frequent groups and biological indices of phytoplankton community of the Santa Cruz reservoir, Rio Grande do Norte, Brazil. The surface water samples of the reservoir were collected monthly during October, 2013 to August, 2014, and environmental parameters were registered and the phytoplankton community of the reservoir was identified. Mean water temperature during the dry period was 29°C and during the rainy season was 24°C. The pH showed a maximum value of 8.9 during the month of October and a minimum value of 6.1 during the month of January. Mean electrical conductivity was high, with a value of 1889 μScm-1 during of the entire study period. The inorganic nutrients maintained very low concentrations during of the study period. The trophic state of the Santa Cruz reservoir indicated an oligotrophic environment. Cylindrospermopsis raciborskii (Cianofícea) and Surirella ovalis (Diatomácea) were present during the months of December to August. During the dry season Cloterium elongatum, Crucigenia tetrapedia and Staurastrum punctatum (Clorofícea) were frequent, while during the rainy season Navícula cuspidata, Surirella ovalis (Diatomácea) and Cylindrospermopsis raciborskii (Cianofícea) were abundant. The phytoplankton community showed higher indexes of richness, diversity, dominance and similarity, in May, after the rainy season, allowing the coexistence of the species. Keywords: phytoplankton; environmental parameters; chlorophyll a; trophic status; reservoir.