Heat shock—induced triploid and diploid Oreochromis aureus were reared in hapas (2 x 1.2 x 0.9 m). Their growth rates were not significantly different (P>0.05) at 24 weeks old after hatching. The genital papillae of triploid fish did not show significant development in comparison with diploid fish. The gonadosomatic indices of female triploid fish were significantly smaller (P<0.01) than those of diploid fish. Non—uniform sizes of sperms were observed histologically in triploid fish. Well—accumulated yolk was pronounced in the oocytes of 24—week old diploid fish. On the other hand, most of the oocytes in triploid fish were oogonia.