Methyltestosterone and estrone were administered orally to fry of the goldfish, Carassius auratus, daily for two months; thereafter, they were raised on a normal diet. All but one of the methyltestosterone-treated fish developed into males, while all estrone-treated fish developed into females. Of seven methyltestosterone-treated males mated with normal females, six produced progeny in a 1 ♀:1 ♂ ratio and the remaining one produced only females. Of eleven estrone-treated females mated with normal males, seven produced progeny in a 1 ♀:1 ♂ ratio and four yielded offspring in a ratio of 1 ♀ : 3 ♂. Induction of sex reversal in both directions by heterologous sex hormones was successfully accomplished. The sex ratios of the progeny of hormone-treated goldfish provide evidence that the male is heterogametic (XY).