Abstract: The pathology of the chronic poisoning of Nonylphenol (NP) in hybrid tilapia（Oreochromis niloticusO.aureus）was studied. Histological observation showed that there were great pathological changes in the tissues of gill, kidney, spleen, and liver from the fish exposed to NP. By NP treatment, the tilapias showed a proliferation of epithelial cells in the gill lamella. The kidney lesions included an atrophy of capillary blood vessels in glomerulus and eosinophilic materials in the tissues. In the spleen, extravasated blood, fibrosis degeneration and massive hemosiderin granules were observed. Liver lesions were characterized by hydropic degeneration, accompanied by haemorrhage in the central veins. Even massive necrotic areas were all found in kidney, spleen and liver exposed in NP.