In this study, micronucleus and nuclear alterations tests were performed on erythrocytes of Oreochromis
niloticus (Perciformes, Cichlidae) in order to evaluate the water quality of the Atibaia river, in an area that
receives effluents discharge of a petroleum refinery and also to evaluate the effectiveness of the treatments
used by the refinery. Water samples were collected in five different sites related with a refinery from
São Paulo State, Brazil. For the micronucleus and nuclear alterations tests, O. niloticus specimens were
exposed for 72 h to the water samples and in pure ground water (negative control). The results herein
obtained indicated that the treatments used by the refinery diminished the cytogenetic damage; however
they were not fully effective, since the final mill has induced damages in the genetic material of the test
organism