The acute toxicity assay must be performed to the limit concentration of 100 mg L−1. If no mortality occurs, it is possible to imply with 99% confidence that the studied organism is not in the most sensible group to the tested substance (OECD, 2008). Even on concentrations ten times higher than the limit concentration, TFB showed no fish mortality. Together with the results of the efficacy trials, it is possible to deduce the drug’s safety by observing its wide therapeutic window. This low toxicity observed in fish is linked to the absence of benzoylurea receptors, which are associated with chitin synthesis in insects .Teflubenzuron presented the same toxicity levels for neotropical and temperate fish (Table 3), independently of exposure time.