This study investigated the physiological effects of a pesticide based on Bacillus thuringiensis (Dipel-WP ®) added in the water and diet of Piaractus mesopotamicus during 24 and 48 h. It was added 0.13 g of de B. thuringiensis per kg of commercial feed; and for the fish subjected to the biopesticide in the water of the tanks, it was added 0.13 g/l of the biopesticide. Plasma levels of sodium, chloride, potassium, cholesterol, glucose, triglycerides, cortisol, total protein, alanine aminotransferase (AlT), aspartate aminotransferase (AST), hematocrit, hemoglobin, erythrocytes number, mean corpuscular volume (MCV) and mean corpuscular hemoglobin concentration (MCHC), number of thrombocytes and leukocytes were determined. Cortisol, lactate, glucose, total protein, cholesterol, triglycerides, AlT, AST, sodium, potassium, hematocrit, hemoglobin, MCV, number of erythrocytes, leukocytes, lymphocytes, eosinophils and PAS-positive granular leukocytes suffered alterations derived from the addition of B. thuringiensis in water and diet of the fish. The toxicity of the concentrations of biopesticide in short-term exposure in water and diet of the fish led to blood alterations (increase or decrease). Therefore, care must be taken to avoid a possible prolonged contamination in the tanks of fish farming by agricultural pollution based on B. thuringiensis.