Effect of fipronil on the histological changes in the gill, liver and kidney of Oreochromis mossambicus were studied at 10% sub lethal concentration (96 hours LC50: 0.006 ppm) for 10, 20 and 30 days. Histological changes such as lamellar fusion, lamellar cell hyperplasia, degenerated secondary gill lamellae; degeneration of hepatocytes and vacuolation; Bowman's space dilation and renal tubules detachment were observed in the gill, liver and kidney respectively, during chronic exposure to fipronil.