The reservoirs of the province of Salta is characterized by its high level of hypertrophy. Monitoring phytoplankton, would enable prevention and control of water quality. We studied the phytoplankton abundance and structure of three reservoirs in the province: Cabra Corral and El Tunal belonging to the upper basin of Juramento and Campo Alegre of the Bermejo's system during a hydrologic cycle (2007-2009). Sites were considered for the areas of greatest influence lotic and the central deeper. In the reservoirs of the upper basin of the Juramento, species diversity decreased during the dry season because of the greater stability in the water column, unlike the phytoplankton density, which tended to be higher. In these reservoirs highlighted the persistence of Cryptophyta. Dinophyta with Ceratium hirundinella as the dominant species, occurred in the three reservoirs in both periods of the hydrologic cycle, in Campo Alegre, was dominant throughout the sample, leading to the lowest values of diversity, richness and evenness. The principal component analysis showed clustering between the sampling sites of each reservoir Campo Alegre differentiating the rest.