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In semi-arid regions, eutrophic waters and stable climatic conditions favor the dominance of cyanobacteria, with frequent blooms. In this study, we hypothesized that the biomass of cyanobacteria will be higher during the dry period and assessed toxic cyanobacteria and cyanotoxins in the Argemiro de Figueiredo Reservoir (Paraiba, Brazil). We conducted 22 samplings between August 2007 and July 2009 at three sites (PC, confluence of tributaries; PNC, net cages; and PD, dam site). Significant seasonal and spatial differences were not observed for abiotic variables; however, reduction in pH, electrical conductivity, alkalinity, and dissolved oxygen occurred only during overflow events in the reservoir. Filamentous cyanobacteria were dominant mainly in the dry period, accounting for 90 % of the total biomass; six of eight dominant or co-dominant species of cyanobacteria were observed to be potential producers of toxins. Planktothrix agardhii (Gomont) Anagnostidis & Komarek was dominant at PC, PNC, and PD and formed perennial blooms, and microcystin-LR was identified in 14 of 16 blooming events. Thus, we suggest that stable conditions, availability of nutrients, and a reduced light regime during the dry period favors blooms of cyanobacteria.