Tadpoles of the differentiated race of R. catesbeiana at standard stages 25-26 were reared at 30° for four months. Tadpoles at 20° served as controls. Histological examination of the gonads at the end of heat treatment showed sex transformation of ovaries toward testicular characteristics in various degrees. The transformation process involved degeneration of oocytes and appearance of rete cells which eventually induced the residual oogonia into spermatogonia and contributed to the formation of seminiferous tubules and vasa efferentia. This process was comparable to the indirect development of the testis of the undifferentiated sex race of the same species. Our findings of heat-induced sex reversal were essentially in accordance with the earlier works of Witschi and Yoshikura. However, some differences have been observed. The rate of sex transformation in this study was much slower than that of R. sylvatica and R. japonica because of the persistence of synaptenes in the ovaries of R. catesbeiana and consequently late appearance of rete cells. The resistance of synaptenes to heat was probably a function of temperature magnitude and/or inherent property of the species. The results support Witschi's theory of antagonism between female and male differentiators.