The composition of the phytoplankton and parameters of the water quality were studied in a tropical reservoir during cyanobacterial bloom episode. Samples were collected at two sites during October 2004. The phytoplankton community consisted of 69 taxa, distributed into 9 classes. Most taxa belonged to Cyanobacteria (17), Chlorophyceae (21), Bacillariophyceae (8), Conjugatophyceae (7), Dinophyceae (1), Chrysophyceae (4), Euglenophyceae (9), Cryptophyceae (1) and Xanthophyceae (1). Anabaena circinalis Rabenhorst ex Bornet et Flahault e Anabaena spiroides Klebahn were the dominant species. Statistical analyses revealed significant spatial variation (p < 0.05) between sampling points only for N compounds and chlorophyll a. Most limnological variables analyzed showed no significant temporal variation (p > 0.05). Cyanobacteria bloom was concomitant with the following environmental conditions, warm water temperatures (19 to 22 oC), periods of water column stability and high concentrations of N (724.9 μg.L-1) and P (90.9 μg.L-1) in the reservoir and NH4 as being the predominant nitrogen compounds. It is speculated that those conditions could play a role in the dominance of cyanobacteria
