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¹ Karnatak University’s Research Laboratory, Environmental and Molecular Toxicology Division, Department of Zoology, Karnatak Science College, Dharwad 580 001, Karnataka, India Viewed : 4224 - Downloaded : 3481 Short-term definitive test by static renewal bioassay method was conducted to determine the acute toxicity (LC50) of commercial grade organophosphate insecticide, malathion (50% EC) in the freshwater edible fish, Labeo rohita. Carp fingerlings were exposed to different concentrations (6.0 to 10.1 μl/L) of malathion for 96 h. The acute toxicity value was found to be 9.0 μl/L and one tenth of LC50 (0.9 μl/L) was selected for sub acute studies. Behavioural patterns and oxygen consumption were studied in lethal (1, 2, 3 and 4 d) and sublethal concentrations (1, 5, 10 and 15 d). Carp in toxic media exhibited irregular, erratic and darting swimming movements, hyper excitability, loss of equilibrium and sinking to the bottom which might be due to inactivation of AChE activity which results in excess accumulation of acetylcholine in cholinergic synapses leading to hyperstimulation. Variation in oxygen consumption (70.39 to 80.50%, -4.45 to 21.35%) was observed in both lethal and sublethal concentrations of malathion respectively. Alterations in oxygen consumption may be due to respiratory distress as a consequence of impairment in oxidative metabolism. Fish in sublethal concentration were found under stress, but that was not fatal. Keywords : Malathion toxicity, behaviour, oxygen consumption, Labeo rohita