Phenthoate is an organophosphorus pesticide widely used insecticide in agriculture sector to control the varieties of pests and ectoparasites. The acute toxicity bioassay tests were conducted at various concentrations ranging from 0.4 and 4.0 mg/L with an interval of 0.4 mg/L on Indian major carp Labeo rohita for 24, 48, 72 and 96 hr. The acute toxicity values were determined as 3.0, 2.6, 2.3 and 2.1 mg/L under laboratory conditions, respectively. These LC50 values showed that the chemical is highly toxic to the fish. Then the fish were exposed to sub lethal concentrations (1/10th of 96 hr LC50) for 1, 4 and 8 days and studied the significant changes in total proteins and glycogen levels of the fresh wet tissues. The total protein content reduced in various tissues like liver, muscle, kidney, brain, gill and gut by the percentage of 22.46%, 17.91%, 12.39%, 8.66%, 8.36% and 8.16% and glycogen content depletion in the order of kidney, liver, muscle, gill, gut and brain tissues by 57.92%, 49.11%, 48.75%, 34.88%, 32.00% and 28.81%, respectively when compared to control group. It was noticed that phenthoate resulted in the gradual decrease of glycogen and protein levels in the vital organs of fish L. rohita, which led increase in the mortality rate.