The present study was aimed to determine the lethality of the insecticide fipronil on two early life stages, post-larvae, and fingerling, of Colossoma macropomum, by determining the lethal concentration 50 (LC50). Five nominal concentrations (0.12; 0.165; 0.21; 0.255 and 0.30 mg/L) and a control in 4 replicates per treatment for 48 hours of exposure were used for the post-larvae; and other nominal concentrations (0.22; 0.27; 0.34; 0.43 and 0.54 mg/L) and a control in 3 replicates per treatment for 96 hours of exposure were used for the fingerling stage. The LC50 value was calculated as 0.22 mg/L and 0.33 mg/L for the post-larvae and fingerling stages, respectively. In addition, the maximum concentration that causes no mortality (MCNM) and the minimum concentration that causes mortality (MCM) for the post-larvae stage were determined as 0.12 and 0.15 mg/L, respectively; and 0.16 and 0.22 ...