Knowledge of the length at first maturity (L-50) is an important tool for fish stock management and conservation. This study estimated the L-50 values of 10 fish species in two reservoirs of the Apodi/Mossoro River in the tropical semiarid region of Brazil. The following species were studied: Curimatella lepidura (Eigenmann et Eigenmann, 1889); Prochilodus brevis Steindachner, 1875; Leporinus piau Fowler, 1941; Leporinus taeniatus Lutken 1875; Triportheus signatus (Garman, 1890); Hoplias malabaricus (Bloch, 1794); Hypostomus pusarum (Starks, 1913); Loricariichthys derbyi Fowler, 1915; Trachelyopterus galeatus (Linnaeus, 1766); Crenicichla menezesi Ploeg, 1991. The captured individuals were identified, separated by species, measured for total length (TL), and categorized as reproductive or immature after dissection. The L-50 values for both sexes were estimated via a logistic regression for each species. Significant differences between values were tested for the species that occurred in both reservoirs (ANCOVA). The estimated L-50 value varied between 7.37 cm (C. lepidura, female) and 19.40 cm (H. malabaricus, female). Only the L-50 value of the males of the species C. lepidura and P. brevis differed between the two reservoirs. This study is the first report of the L-50 for seven species and the first report for tropical semiarid river basins for two species. These results are important for fish resource management and species conservation.