The gross anatomy and the internal structure of the marmoset (Callithrix jacchus) brain stem, cerebellum and diencephalon are described based on the observations made on the material obtained from 10 adult animals (native name: sagui). 5 brains were used for gross anatomical observations; the other 5 brains, included in toto in celloidin, were cut into serial sections, properly stained and used for comparative studies of the internal structure. Only general descriptions are given considering this part of the encephalon because, in further papers, it is intended to go on describing in details the nervous system of this small primate, very common in Brazilian jungles and easily domesticated. It shows to be a very good and inexpensive animal for neurophysiological experiments.