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A detailed study on the structure of the gonad of Monopterus albus was made as a basis for analysis of gonadal steroids in this sex-reversing teleost. Two types of males were identifiedand their existence appeared to be a result of the difference in gonadal ontogeny among the individuals in natural populations. The germinal area of the gonad, the gonadal lamellae, exhibited à zoned nature with regard to the location of the female and male germ cells. Observations suggested that the male germ cells originated from gonia pre-existing in the inner zone of the gonadal lamellae before sex reversal. Natural reversal of sex in this protogynous hermaphrodite was found to be usually a postnuptial event and was always accompanied by loss of ovarian tissue and by development of interstitial (Leydig) cells. In the mesogonial region of the gonadal wall, peculiar mesenchyme cells were found, their significance remained uncertain.