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We assessed which environmental variables had the greatest influence on the occurrence of alien mollusks in reservoirs in the semi-arid region of Brazil during periods of drought. The study was conducted in the Argemiro de Figueiredo, Epitacio Pessoa, and Pocoes reservoirs in the Paraiba River watershed in northeastern Brazil during July and October/2015. Sampling was undertaken in three regions in each reservoir: 1) near the inflow from the principal tributary; 2) the mid-reservoir section; and, 3) near the dam. Three sampling sites were established in each reservoir region. The physical and chemical parameters of the reservoir waters in each site were measured and the granulometric compositions of their sediments determined. A total of 8635 benthic individuals were identified, of which 6569 were alien mollusks. Alien mollusks represented 90 % of all individuals collected from the benthic community in the Argemiro de Figueiredo reservoir, 63 % in Epitacio Pessoa, and 35 % in Pocoes. Melanoides tuberculata (6283 individuals) was the most abundant mollusk species, followed by Corbicula largillierti (286 individuals). Significant differences were observed in terms of the abundances of alien mollusks in the different reservoirs (p= 0.0001) and during the different sampling periods (p=0.0001), but not among the different sampling sites in each reservoir (p=0.937). In terms of environmental parameters, significant differences were observed in the different reservoirs (p=0.0001), but not in the different sampling sites in each (p=0.7428) or during the different samplings periods (p=0.346). The Best model (R-2=0.61) selected five predictor variables for the occurence of alien mollusks: total nitrogen, salinity, temperature, gravel, and silt. Environmental variables, together with periods of severe drought and the natural characteristics of the region (such as the mineral composition of the soils) favored the occurence of alien mollusks in semi-and reservoirs.