The alterations in the epithelium of the gill filaments and the secondary lamellae of the gills of Cirrhinus mrigala, on exposure to “Nuvan,” have been explored in the present investigation using light and scanning electron microscopy. The fishes were exposed to two sublethal concentrations, 5？mg/L and 15？mg/L, of “Nuvan.” The changes are more rapid and intensive at higher concentration than at lower concentration, suggesting that the changes are dose dependent. Increase in thickness of epithelium covering secondary lamellae, merger of epithelium of gill filaments and adjacent secondary lamellae, and aneurysm is considered to reduce efficiency of gills for gaseous exchange. A significant decline in the density and area of the mucous goblet cells in the epithelium of the gill filaments and the secondary lamellae of C. mrigala exposed to “Nuvan” could be correlated with excessive loss of the secretory contents of these cells, uncompensated by their production in sufficient quantities. The histopathological changes, in general, take longer time to recover in the fishes exposed to 15？mg/L than those exposed to 5？mg/L indicating that the changes in fishes exposed to higher concentration are more severe than those at lower concentration of the insecticide. 1. Introduction The use of chemical insecticides is fairly recognized as a cost effective method of controlling the pests and parasites in agriculture and aquaculture practices, but these chemicals are highly toxic to other species in the environment. They produce adverse effects on nontarget aquatic organisms living in areas near agricultural fields [1]. Pesticides often end up in aquatic habitats carried up by wind, rain water, or through uncontrolled waste disposal. Being biodegradable and of short persistence in environment, organophosphorus insecticides are extensively used for the control of pests in agricultural fields, and, therefore, large quantities of these insecticides reach the water bodies [2]. “Nuvan,” an organophosphorus insecticide, is released on surface water as it is a commonly used chemotherapeutant in fish farming to eradicate crustacean ectoparasites [3–7] and to treat culture ponds for the eradication of freshwater fish predators prior to the stocking of spawn—fry, fingerlings, or juveniles of carps [8–10]. In fish, gills are the main site of gaseous exchange [11–15]. In addition, they are involved in osmoregulation [16–19], acid base balance [20–23], and excretion of nitrogenous compounds [24–26]. The complexity and constant contact with the surrounding water make the gill the first
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