The evaluation of the alochthonous and cosmopolitan mosquitofish species Gambusia affinis suitability as a bioindicator species and the induction of its liver cytochrome P450-dependent mixed function oxygenase (MFO), measured as the 7-ethoxyresorfin-O-deethylase (EROD) activity, as well as changes in DNA integrity, measured by the Fast Micromethod, for the monitoring of organochlorine fresh water pesticide contamination, were the main aims of the study. The test mosquitofish were exposed under laboratory conditions to several doses (0.1, 10 and 100 microg l(-1)) of lindane in experimental basins for up to 7 days, and a subsequent field study was carried out at five natural ponds in the south-western Istrian peninsula, Croatia, where up to 10 fish were collected from each pond. Results obtained during the studies showed positive correlations between the measured biomarkers in G. affinis liver (EROD activity and DNA integrity status) and lindane (laboratory experiment) or persistent organochlorine pollutant amounts in natural pond sediments (field study). The clear dose-responses of EROD activity and DNA integrity deterioration in G. affinis were recorded after exposure to 0.1-10 microg/l lindane and 96 h exposure to lindane, respectively. The results indicate that the mosquitofish G. affinis, due to its biological-ecological characteristics and the biomarker dose-response, is suitable for the monitoring of fresh water organochlorine pesticide contamination in general and lindane in particular.