As aquatic organisms have their outer bodies and important organs such as gills almost entirely exposed to water, the effect of toxicants on the respiration is more pronounced. Pesticides enter into the fish mainly through gills and with the onset of symptoms of poisoning, Profenofos, a well-known organophosphate pesticide has been in agricultural use over the last two decades for controlling pests of paddy, cotton and tobacco. In the present study, an attempt has been made to study the effect of profenofos on oxygen consumption and gill histopathology of the Indian major carp, C. catla. The fishes were exposed to sublethal concentration (1/10th and 1/20th of 96 h LC50) of profenofos for a period of 15 days. The treatment of these profenofos brought about significant decrease in oxygen consumption as compared to control. Exposure to profenofos was found to result in several alterations in the histo-architecture of the gills of C. catla. The alterations included curved secondary gill filaments, necrosis of gill filaments and conjestion of Secondary Lamellae. The significance of this study as a bio-indicator for assessing the toxicity and economic importance of the fish is discussed.