In the present study, the effects of long term exposure (5 and 10 days) of 100 µg/lit 4-NP (nonyphenol) on the testis were investigated in African catfish Clarias gariepinus (Burchell, 1822). Histological examination of the testis of fish treated with 100 µg/lit 4-NP for 5 days showed the disintegration of cysts, separation of cells within the cysts, hypertrophy of sertoli cells and vaccuolation in testis. Histological examination of the testis of fish exposed to 100 µg/lit for 10 days showed alteration in structure of the primary spermatocytes. The structure of the spermatocytes changed from spherical to sickle shaped. Hypertrophy of sertoli cell, severe destruction of germ cells (spermatogonia), and vacuole formation was also seen. The study indicated that 4-nonylphenol had marked effects on the histology of testis of C. gariepinus. The severity of effects of fish increased with the time of exposure and it was noticed that there were marked structural changes in the testis exposed to 4-Nonylphenol for long term exposure.