The classical view of mammalian sexual differentiation is that a gene on the Y chromosome transforms the indifferent gonad into a testis. The Leydig cells then secrete androgen which stimulates the development of the male reproductive tract, and the Sertoli cells secrete Mullerian inhibitory substance which inhibits the development of the female reproductive tract. In the absence of a testis, the Mullerian duct develops into the Fallopian tubes, uterus and vagina. Thus the whole of sexual differentiation is thought to be hormonally mediated as a consequence of this initial genetic determination of gonadal sex. We have found evidence in a marsupial mammal for extensive sexual dimorphisms which precede any morphological differentiation of the gonads. Thus the classical view of mammalian sexual differentiation may have over-emphasized the role of testicular hormones, and overlooked earlier genetic effects.