Fenthion is an organophosphate pesticide, widely used all over the world as a broad-spectrum insecticide for the protection of numerous crops especially rice. Due to these agricultural activities Fenthion has repeatedly been detected in surface waters and soil. Thus. Fenthion is among the most toxic pesticides for aquatic life especially fish. This study had investigated die toxic effect of Fenthion on some immunological and histopathological parameters. Fish were exposed to concentrations of 0.00 (control). 0.S3 ppm and 2.1 ppm for 96 hours. The histopathological alterations for lymphoid and non-lymphoid organs, as compared to the control group. included hemosiderosis and atrophy in spleen and pronephrous. In gills, hemorrhage, progressive degeneration and lymphocj le infiltration were reported: in liver and pancreas atrophy has been shown in addition to excessive zvmosen granules secretion in pancreas. The kidney trunk showed glomerular and tubular atrophy and lymphocyte inilltration. For the alterations of plasma immunoglobulin level. Fenthion at all concentrations suppressed plasma IgM level (P< 0.05). So it can be concluded that Fenthion causes adverse effects on immunological and histopathological parameters of exposed fish.