Norflurazon is a bleaching, preemergence herbicide. Due to its mobility and long half-life it presents a potential for groundwater contamination. The aim of our study was to investigate toxic effects of norflurazon on non-targeted aquatic bioindicator organism, the planarian Polycelis felina (Daly.). Animals were exposed to water solutions of norflurazon in concentrations 200, 20, 2 and 0.2 microM. Mortality, locomotive and morphological changes were monitored. Histological changes were studied both on treated and control animals with light microscopy. The primary DNA damage on single planarian cells was studied using the alkaline comet assay. Three comet parameters were studied: tail length, percentage of DNA in comet tail and tail moment. The results showed that norflurazon caused mortality, locomotive, morphological and histological changes in treated animals compared to corresponding controls. The most prominent histological changes were damage of the outer mucous layer, lack of rhabdites, damage to epidermis and extensive damage to parenchyma cells. The results of alkaline comet assay indicated that norflurazon in concentrations of 2 and 0.2 microM induces significant increase of primary DNA damage in planarian cells compared to the corresponding control animals. The mean values of all three measured parameters were significantly elevated on the fourth day of the treatment compared with the first and the seventh day. Based on the results of mortality and locomotive observations, we conclude that the fourth day of the treatment represents a certain threshold within planarian metabolism followed by the beginning of detoxification and recovery. However, histological preparations and comet data statistics show results indicating that high toxicity on the seventh day of the treatment gave the results of decrease of DNA damage due to the tissue/cell damage (apoptosis) and not recovery. The present study showed the ability of norflurazon to induce a wide range of different toxicological responses in freshwater planarian Polycelis felina (Daly.).