The mutagenicity of interstitial water and organic extracts from the sediments in the Cadeia and Feitoria Rivers, RS, Brazil, were evaluated by Salmonella microsuspension bioassay using TA97a, TA98, TA100 and TA102 strains, in the absence and presence of S9 mix. At the contaminated site, the mutagenic responses for interstitial water, suggested the presence of frameshift and base pair substitution mutagens, including oxidative substances. Organic extracts presented direct or indicative mutagenesis to the TA97a, TA98 and TA100 strains. In general, an exogenous metabolic systems decreased the mutagenicity of the samples. High concentrations of total chromium found in the sediment and interstitial water as well as total mercury in the sediment of the contaminated site, when compared to the control area, may help explain the mutagenic results. The livers of Gymnogeophagus gymnogenys collected in this impacted area, compared to a non-polluted site, were analyzed for oxidative stress parameters. Compared to the controls, there was a significant increase in the activity of superoxide dismutase (SOD) at levels of substances reactive to thiobarbituric acid (TBARS), and in the chemiluminescence of hepatic cells in fish in the polluted area. The concentration of cytochromes P450 and b5 decreased drastically in the fish at the polluted site, while the catalase activity did not change. It was possible to correlate the biological changes in the fish with the presence of mutagenic compounds in sediment and interstitial water in this area.