The study aimed mainly to assess effects of omethoate on certain oxidative stress biomarkers in various tissues of frogs (Rana ridibunda). Biomarkers selected for stress monitoring were malondialdehyde (MDA) and antioxidant defense system (ADS) constituent such as reduced glutathione (GSH) level, glutathione peroxidase (GSH-Px), glutathione-S-transferase (GST) and superoxide dismutase (SOD) activities in the liver, brain, heart and kidney tissues of frogs exposed to 10 and 20 ppm dosages of omethoate for 24, 48, 72 and 96 hours. Results showed that the administrations of omethoate promote MDA content in the tissues of the frogs treated with both dosages of omethoate. With regard to the ADS, GSH-Px, GST, SOD activities and GSH levels decreased after 24, 48, 72 and 96 hours in the tissues of treatment groups compared with controls. Collective results demonstrated that the exposures to omethoate of frogs induced an increase in MDA joined with fluctuated ADS. This may reflect the potential role of these parameters as useful biomarkers for assessment of aquatic pollution.