The aim of this study was to investigate the effect of an organophosphorus insecticide omethoate (OM), on certain oxidative stress biomarkers (malondialdehyde (MDA), reduced glutathione (GSH), glutathione-S-transferase (GST), glutathione reductase (GR), catalase (CAT)) in tongue, lung, stomach and muscle tissues of adult frogs (Rana ridibunda Pallas). Animals were exposed to 10 and 20 parts per million dosages of OM for 24, 48, 72 or 96 h. According to the results, MDA level increased significantly in lung and stomach tissues. GSH content fluctuated in lung and muscle while it elevated in tongue and stomach tissues. With regard to antioxidant enzymes (GST, GR and CAT), their activities reduced in tongue, while they increased in lung and fluctuated in stomach and muscle tissues. It can be concluded that exposure of frogs to OM are characterized by increased MDA levels and fluctuated enzyme activities and GSH contents. This may reflect the potential role of these parameters as useful biomarkers for assessment of OM toxicity.