The histopathological effects of deltamethrin on the gill, liver and gut tissues of the mosquitofish, Gambusia affinis were determined by light microscopy. The fish were exposed to two sublethal concentrations of deltamethrin (0.25-0.50μg/l) for periods of 10, 20 and 30 days. The most common gill changes at all doses of deltamethrin were desquamation and necrosis. Besides, epithelial hypertrophy, lifting of the lamellar epithelium, oedema, dilatation of the capillaries primary lamellae, aneurism, epithelial hyperplasia and fusion of the secondary lamellae were other histopathological effects. Hepatic lesions in the liver tissues of fish exposed to deltamethrin were characterized by hypertrophy of hepatocytes, significant increase of kupffer cells, circulatory disturbances, focal necrosis, fatty degeneration, nuclear pycnosis and narrowing of sinusoids. Infiltration of mononuclear leucocyte and eosinophils towards lamina propria, necrosis were detected in gut tissues of fish after exposure to deltamethrin.