Resumo. Polycyclic aromatic hydrocarbons are one of the most important organic pollutants in environmental studies. The aim of this study was to assess the naphthalene acute toxicity in two fish species, Astyanax lacustris (LLcust, 1875) and Geophagus brasiliensis (Quoy & Gaimard, 1824). The fish were exposed to naphthalene (0.005, 0.03, 0.3, and 3 mgL-1) in water and after that the piscine micronucleus test in erythrocytes, comet assay in blood, liver and gill cells, glutathione S–transferase (GST) activity in the liver, and accumulation of naphthalene in the bile were performed. The susceptibility of the two species was similar and naphthalene was not genotoxic in all tested tissues. The liver GST activity may have been responsible for less damage observed in the liver while the highest DNA damage occurred in blood cells. However, low concentrations of naphthalene in water can stimulate apparent benefits, such as less DNA damage, which would be a compensatory response to an imbalance of homeostasis. The naphthalene is absorbed and can accumulate in the gall bladder, a greater accumulation of PAH was observed in A. lacustris, while G. brasiliensis did not differ from the control. The naphthalene concentrations are not genotoxic to the tested species, although they can potentially accumulate into the body. Keywords: Comet assay. Ecotoxicology. Fish. Genotoxicity. Hormesis.