The aim of this study was to determine the acute toxicity and sublethal effects of methyl parathion on tambaqui (Colossoma macropomum). For this, juveniles of tambaqui (45.23  0.43 g and 11.91  0.08 cm) were transferred to tanks of 50 L and acclimated for 48 hours. For testing the acute toxicity the tambaquis were exposed to seven concentrations of methyl parathion (0.0; 1.0; 2.0; 3.0; 4.0; 5.0 and 6.0 mg L-1), with three replicates, for 96 hours to determine the lethal concentration (LC50). The water quality parameters were monitored during the assays and the feeding suspended in this period. After the determination of the LC50 fish were exposed to sublethal concentration and were evaluated the physiological responses by glucose and hematological analyses. The behavioral changes of the fish after the application of methyl parathion were: increase of opercular movements, erratic swimming, and exophthalmy. The LC50 96 hours of methyl parathion for tambaqui was calculated to be 2.91 mg L-1, indicating low tolerance of tambaqui to this organophosphorous. Although, the exposure of tambaqui to sublethal concentrations did not promote alterations in glucose and hematological indicators, that provided evidence of disturbances in the homeostasis of the species.