This research investigated the effect of Pawpaw (Carica papaya) seed meal (PSM) on haematological parameters and gonad histology of Oreochromis niloticus (Linnaeus, 1758). Freshly hatched fry of O. niloticus were fed blended commercial diet (35%CP) mixed with varying levels of PSM (0, 2, 4, 6 and 8gkg^-1) for 28days in fifteen aquarium tanks (30×30×20cm³). Resultant fingerlings were maintained in outdoor concrete tanks (1×1×1m³) for 140days till they gained an average weight of 30g. Haematological analysis and gonad histology was done to determent the effect of the PSM on the fish. Haematological changes observed in this study did not revealed detrimental effect of feeding of PSM. However, histological evaluation of the gonads showed various degrees of deformities and tends to increase in severity as the levels of PMS increased. It was concluded that PSM might not affect the health status of the Nile tilapia but lead to sterility of the fish.