Toxicity tests revealed that the Indian catfish, Heteropneustes fossilis, is highly sensitive to cypermethrin, a synthetic pyrethroid. The LC50 value decreased with increasing exposure time, revealing a time-dependent action of cypermethrin. Biochemical studies confirmed this, showing that the effects on carbohydrate metabolites also revealed a time-dependent response. The probable reasons for the disturbance in the homeostatic mechanisms of carbohydrate metabolism are discussed.