This paper reviews current knowledge concerning the endocrine and environmental regulation of gonadal sex differentiation in gonochoristic fish. In gonochoristic fish, although potentially active around this period, the hypothalamo-pituitary axis is probably not involved in triggering sex differentiation. Although steroids and steroidogenic enzymes are probably not the initial triggers of sex differentiation, new data, including molecular approaches, have confirmed that they are key physiological steps in the regulation of this process. Environmental factors can strongly influence sex differentiation in gonochoristic fish. The most important environmental determinant of sex would appear to be temperature. Interactions between environmental factors and genotype have been suggested for gonochoristic fish.