The present study was oriented toward assessing the effects of aflatoxin on O. niloticus with particular reference to clinical manifestations and suppression of macrophage phagocytic activity. Ten day LD 50 of intraperitoneal administered crude aflatoxin was 0.5mg / kg bw. Long term effects of levels of chronic aflatoxicosis included mortality (63.5 %), growth suppression and reduced macrophage phagocytic activity. Changes in macrophage phagocytic activity were observed even in the absence of clinical manifestations, hence phagocytic assays may be useful as an early warning indicator of environmental toxicosis.