The Glutathione S-Transferase (GST) activity has been proposed as a biomarker of susceptibility to the presence of potentially damaging xenobiotics in aquatic organisms. The aim of this work was to measure GST activity in the liver of Sciades herzbergii (catfish) in order to evaluate the biochemical effects of pollutants. The catfish samples were collected along known pollution gradients areas (A1) and from areas regarded as relatively free of anthropogenic input (A2), in São Marcos Bay, São Luis de Maranhão, Brazil. The variables analyzed in fish were: length, weight, gonadal stages, gonadosomatic index and GST activity. The databases from this analysis were compiled, and generalized linear models were used to analyze the dependence of enzyme activity on the areas of sampling and on selected biological parameters of fish. A significant difference was observed in GST activity in the liver of S. herzbergii in the comparison between fish from the contaminated site and those from the reference site (P < 0.05). Morphometric (length and weight) parameters and gonadosomatic index of collected fish were significant in the linear model of GST activity only in the reference site. These results may be due to the activity pattern of the enzyme, which increases with the sexual maturity of the animals in healthy environments. In the contaminated area (A1) these correlations do not exist, probably as a result of the energy used in the biotransformation of the various contaminants.