Gills are vital structures for fish, since they are the main site for gaseous exchange as well as partially responsible forosmoregulation, acid-
basic balance, excretion of nitrogenous compounds and taste. Chemicals in the water may alter the
morphology of branchial cells of fish and are the useful model for environmental impact and ecotoxicology studies. In order toinvestigate the effects of an organoph
osphorous compound, malathion, on the gills of the fish,Glossogobius giuriswereexposed to sublethal concentration (0.05, 0.25 and 0.5ppm) for 24, 48, 72 and 96 hr. Through light microscopy, shrinking of
the branchial epithelium, followed by detachment a
nd hyperplasia were observed. Externally, the branchial filaments presented the gradual disappearance of microridges. Even in higher sublethal concentration (0.5ppm), the organophosphorous pesticide reduced the health and fitness of the fish, as consequenc
e of secondary effects derived from changes in the secondary epithelium, cellular size of chloride, mucous and pillar cells, cell diameters and significant shape and tissue damage was
noticed
