A developmental study of the Telencephalon of the trout (Salmo irideus) has been done. The stages of fixation were 18 days after fecondation, hatching; 5 days, 1, 2, and 3 months, and 1 year after hatching. The different cell-masses are summarized in table 1. In the young trout, eversion is not important. Just, 2 olfactory bulbs are evaginated. In front of the commissura anterior, we can see: the Nucleus ventroventralis and the Nucleus ventrodorsalis, on the one hand; a voluminous dorsal area which includes: the Nucleus dorsolateralis, the Nucleus dorsomedialis, the Nucleus dorsocentralis and the dorso and ventro-lateral groups, on the other hand. The different Nuclei of the dorsal area are differentiated from a primordial territory which is the area intermedius at the hatching stage. On the hemispheric wall at the level of the tela, we can see the Nucleus teniae. Behind the plan of the commissura anterior, the Nucleus posterior is already seeing at the end of the first month after the hatching. A Golgi-Cox study showed some aspects of different kinds of neurons, and an important neuropil at the level of the Nucleus dorsocentralis too.