In the present investigation  the effects of Dimethoate insecticide on survival chance (acute toxicity), behavioral response and blood biochemical and haematological parameters and the expression of isoenzymes in Catla catla (Hamilton) and Labeo rohita (Hamilton) after exposure to sublethal concentrations of Dimethoate was studied. It is evident that no mortality of Catla catla and Labeo rohita was recorded at 20.5mg/l Dimethoate up to 24 hrs. of exposure. At concentrations of 23.0, 23.5 and 24.0mg/l of Dimethoate 100% mortality of Catla catla was recorded at three different concentration of Dimethoate viz., 23.0mg/l, 23.5mg/l and 24.0mg/l in 96 hrs. of exposure. The LC50 values of Dimethoate at various exposure times were 21.0mg/l for about 84 hrs; 21.5mg/l for 72 hrs; 22.0mg/l for about 60hrs; 22.5mg/l for 48 hrs; 23.0mg/l for 48 hrs; and 23.5mg/l for 24 hrs. The percentage mortality of Catla catla increased with increased concentration of Dimethoate and with decreased exposure time. Similarly at concentrations of 23.0, 23.5 and 24.5mg/l of Dimethoate 100% of mortality of Labeo rohita was recorded. Catla catla and Labeo rohita showed behavioral alterations against Dimethoate intoxication. Uncoordinated movements, erratic swimming, convulsions, excess mucus secretion, decreased opercular movements, loss of balance, drowning and change in body pigmentation, muscle fasciculation. Moribund lethargy, refusal of feeding, respiratory distress became more apparent with increase in duration of exposure at all test concentration of Dimethoate. The glycogen content, total proteins and total lipids decreased in the blood of Catla catla and Labeo rohita under the toxicity of Dimethoate whereas the levels of total free sugars and total free amino acids increased. Dimethoate caused reduction in total proteins in the blood of both Catla catla and Labeo rohita. The acute exposure to Dimethoate resulted in a significant decrease in count of developmental forms of myeloid sequence and the segmented neutrophilic granulocytes in the experimental group. Catla catla and Labeo rohita exposed to acute effects of the Dimethoate insecticide showed significantly (P < 0.01) decreased concentration of alkaline phosphatase and significantly (P < 0.01) increased concentration of ammonia, aspartate aminotransferase, lactate dehydrogenase,creatine kinase and lactate in blood plasma. The main biochemical blood profile response of Catla catla and Labeo rohita to the acute effect of 23.5 mg/l of Dimethoate was a significantly (P < 0.01) decreased concentration of alkaline phosphatase and significantly (P < 0.01) increased concentration of NH3, LDH, AST, CK and LACT in blood plasma.

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