The purpose of this study was to evaluate the adverse effects of sublethal doses of fenitrothion, an organophosphothionate insecticide on brain, gill, liver, and muscle tissues as a ratio of 8-OHdG to dG to indicate the DNA damage and erythrocyte micronucleus frequency for genotoxicity of carp (Cyprinus carpio L.) fingerlings. In our study, the mean weights and lengths of the fish (n = 4-12) were 31.13 +/- 14.24 g and 12.53 +/- 1.41, respectively. Before the experiment, fish were maintained in aerated dechlorinated tap water at 21.8 +/- 1 degrees C and fed daily with commercial feed at a rate of 2% of their body weights. Experiments were conducted under static conditions in the aquaria. Technical grade (95%) fenitrothion was diluted in acetone to give a dosing solution of 10 mg/L. The increased lesions/10(6) DNA bases (p < 0.05) of liver tissue of exposure group (0.49 +/- 0.18) was observed when compared to control group (0.28 +/- 0.30). There was not any significant differences between brain tissues, no damage were detectable in gill and muscle tissues of control groups, and in exposure groups altered levels of damage were detected for gill (0.06 +/- 0.05) and muscle (0.16 +/- 0.21) tissues. The increased micronucleus frequencies (%) in erythrocytes of carp following the exposure to 48 h fenitrothion (6.43 +/- 3.89; p < 0.05) was observed when compared to control group (1.29 +/- 1.03). The available data indicate that there is still lack of well-established dose-response relationships between occupational or environmental exposures and the induction of 8-OHdG. Such biomarkers may be used in assessing adverse/toxic effects of pesticides as environmental stressors.