Reproductive features of Schilbe mystus were studied in Oyan Lake, (the first man-made lake in Ogun State, Nigeria) from January to December, 2001 in order to establish its reproductive biology. Gonad maturation was determined macroscopically and microscopically, fecundity by gravimetric method and gonadosomatic indices calculated. Six maturity stages occurred in the species in the lake. Histological structures of the ovaries revealed that the diameter of oocytes of immature / stage I ranged from 0.03 to 0.09mm (mean= 0.06mm ± 0.031). Maturation took place after the oocytes have attained a maximum diameter of 0.66mm in ripe/ stage IV gonad of fish with a total length of 24.9cm. Fecundity of S.mystus in the lake ranged from 3,500 - 9,700 for fish with a total length of 13.4-24.9cm. The species is a total spawner as there was one major peak on the frequency polygon distribution of egg diameters. There was a high correlation between gonadosomatic index (GSI) and amount of rainfall (r = 0.7807) as well as between the percentage of female with ripe gonads and amount of rainfall (r = 0.5566) in the lake. These indicate that the species bred during the rainy season. The females were significantly more numerous than males (p<0.05) during sampling period.