Male tilapia fish (Oreochromis niloticus) and water bugs (Lethocerus niloticum) were studied in a presence of malathion (diethyldimethoxy thiophosphoryl thiosuccinate) of commercial grade 57%. Fish were fed during the experimental period once daily on a commercial dry pellet ration, while water bugs were fed on fish offales. They all were acclimatized to the same laboratory conditions for 2 weeks before the beginning of the experiment. Different malathion concentrations were experimented. The present study revealed that the minimum malathion concentration level that induced mortality of water bugs without any lethality on Oreochromis niloticus fish was 0.26 mg/L for 96 hrs exposure period. However, some toxic effects and histopathological alterations were observed at that condition. Gonadosomatic index, serum Testosterone, T3 and T4 concentration levels were determined. Brain AChE and LDH activities were also analyzed. Fish liver, spleen, gills, testes, brain and thyroid gland samples were histopathological studied.