The mechanism of sex determination in Buergeria buergeri (SCHLEGEL) was studied by making use of sex-reversed females. There was nearly an equal number of one-nucleolate and two-nucleolate tadpoles in those produced from two pairs of males and females just as in the tadpoles collected from the field. The one-nucleolate and two-nucleolate tadpoles collected from the field as well as those raised in the laboratory were administered estrogen or androgen in order to obtain sex-reversed frogs. Masculinization occurred in a greater or less degree in many of one-nucleolate females administered estrogen or androgen at the tadpole stage. Seven of them administered estrogen or androgen attained sexual maturity. Offspring were obtained from two mature sex-reversed genetic females administered estrogen. Feminization also occurred to a certain degree in many of the juvenile two-nucleolate males administered estrogen at the tadpole stage. Of the 1067 offspring obtained from the two mature sex-reversed genetic females (1-nu) mated with three normal females (1-nu), 243 (22.8%) had no nucleolus, 525 (49.2%) had one nucleolus, and 299 (28.0%) had two nucleoli, when examined at the neurula, tail-bud and tadpole stages. While all the anucleolate individuals did not live beyond the tail-bud stage, the one- and two-nucleolate individuals mostly survived normally and became females and males, respectively. It was confirmed that the sex chromosomes are of the ZZ/ZW-type in Buergeria buergeri and that the nucleolar organizer is situated in the Z chromosome and inherited as a Mendelian character.