The expression of the engrailed homeobox gene during trout embryogenesis has been examined using immunohistochemistry and the monoclonal antibody 'Mab 4D9'. Engrailed has been suggested to play an important role during development by controlling position-specific characteristics in the CNS of the early embryo. In the present study we have analyzed the expression of engrailed at 5 stages of embryonic development of the trout (Salmo fario L.). The earliest stage analyzed was when the optic vesicles appear. Engrailed was then expressed in the posterior mesencephalon and anterior metencephalon, in a caudorostrally decreasing gradient. As the embryo develops, the pattern of the engrailed expression increases in spatial complexity. Thus, in the later stages of development, just before hatching, engrailed was found in hypothalamic areas, the germinative matrix layer of the cerebellum, the mesencephalic tegmentum, the caudal optic tectum and in the area of the trigeminal motor nucleus. This is similar to the distribution of engrailed in embryos of amphibians, birds and mammals.