This study ascertains the length-weight relationship and predicts body composition equations in cultivated Astyanax aff. fasciatus. A total of 300 fish were distributed in five groups according to body weight: 1.19 ± 0.18, 3.7 ± 0.70, 7.55 ± 0.97, 12.28 ± 2.32 and 22.16 ± 2.64 g. The length-weight relationship was elaborated using linear (y i = b0 + b1x i) or second-order (y i = b0 + b1x i + b1x i²) regression analysis. The value of the b slope in the length-weight relationship was 3.6971 and the intercept was 0.0031. The prediction equations obtained for body moisture, crude protein, crude lipid and ash were, y = 71.680 - 0.404BW, y = 17.140 - 0.095BW, y = 8.432 +0.364BW and y = 3.720 - 0.032BW, respectively, where BW is the body weight of fish (g). The use of prediction equations to describe body composition as a tool to support fish production and commercialization is suggested.