In order to determine the teflubenzuron acute toxicity, fasted fish(45 tilapias and 45 pacus) were exposed to concentrations of 700.0,800.0, 900.0 and 1000.0 mg TFB L−1 . Also, a control with no application was installed. The tests were performed in triplicate, three fish per trial, in a static system, in 3 liters aquarium filled with water The mortality rate, the clinical signs for drug toxicity and the water physical-chemical parameters−pH, temperature (°C), electrical conductivity (μS cm−1) and dissolved oxygen (mg L−1)−were monitored at 0, 24 and 48 hours after treatment. The TFB toxicity was classified according to Zucker (1985