[en] The present study assessed the composition of the phytoplankton community and the physicochemical variables in an area located within the ore treatment unit - Brazilian Nuclear Industries, in Caldas and also in 'Antas' dam, which is under the influence of the treatment unit. Water samples were taken from three sites; one located within the treatment unit (site CM), which receives non-treated effluents generated during the mining process; and the other sites are located in 'Antas' dam (sites Cab and 41). We determined the values of dissolved oxygen, pH, chlorophyll a, hardness, thorium, uranium, sulfate and total organic nitrogen in water samples, and identified the phytoplankton community in October 2008 and January 2009. Water samples from the site CM exhibited lower pH medium values (3.9) than from the site 41 (6.9). The highest medium value of chlorophyll a was detected in water samples at site CM (5 μg L^-1), whereas the lowest value was recorded at site 41 (0.47 μg L^-1). Higher medium values of sulfate were detected in water samples from site CM (1743 mg L^-1) compared to site Cab (110.11 mg L^-1). We identified six classes in the phytoplankton community at site CM and eight classes at sites Cab and 41. Total average density of phytoplankton were 444 ind mL^-1, 316 ind mL^-1 and 303 ind mL^-1 at points Cab, 41 and CM, respectively. The results obtained show that the environmental conditions at site CM are not favorable to the maintenance of a high density in the phytoplankton community. (author)