The development of the cephalic region of rainbow trout in the 24th, 30th and 36th stages, corresponding to the table of development by Vernier (1969), was studied using light microscopy and SEM. The 24th stage shows a voluminous cephalic region as a principal feature. At this time, most components of this region have started their differentiation. The 30th stage can be considered as an intermediary phase in the embryonic formation, and it is characterized by the high degree of differentiation between the nervous system and the sense organs. In the 36th stage, the basic final shape of the embryo has been reached. The results of this study show that the differentiation of gill lamellae and the opening of the mouth takes place later than that moment described by other authors. This suggests that development between 30th and 36th stages is a critical survival period for rainbow trout, in which respiratory and digestive functions start.