The diencephalon of the vervet monkey (Cercopithecus aethiops) is described and compared with that of other primates, particularly the macaque monkey (Macaca mulatta). In the vervet monkey, the thalamus is divided into six nuclear groups: anterior, midline, medial, dorsolateral, ventrolateral and posterior. The anterior nuclear group shows a regressive nucleus anterodorsalis and a poorly defined demarcation between the nuclei anteromedialis and anteroventralis. There is a large interthalamic adhesion in which the ventral region containing nuclei centralis medialis, interventralis and reuniens, shows more regression than the dorsal region consisting of nuclei parataenialis and paraventralis. As the nucleus medialis ventralis is not present, the nucleus medialis dorsalis is described as the nucleus medialis; this is clearly differentiated into three parts, each part showing different cytological and architectonic features. The nucleus centrum medianum is larger and better developed than that of lower primates but is not entirely demarcated from the nucleus parafascicularis. The pulvinar is an enormous outgrowth from the dorsolateral thalamic nuclei. The ventrolateral thalamic nuclei are well differentiated from one another: a distinct nucleus ventralis dorsomedialis is present and appears to possess definite topographical and cytological differences from those of the other ventrolateral nuclei. The nucleus ventralis posteromedialis is further differentiated into a magno- and a parvocellular part. The nucleus ventralis posteroinferioris is a distinct entity. The lateral geniculate body shows a definite differentiation into two nuclei: pregeniculate and lateral geniculate nuclei. The lateral geniculate nucleus is a six-layered structure which is of the inverted type; it has undergone a lateral rotation through an anteroposterior axis from a dorsal position in the prosimian thalamus to its present ventral position in the higher primate thalamus. The medial geniculate body is not laminated as is the lateral geniculate nucleus; it consists of a small dorsomedial magnocellular and a large ventrolateral parvocellular part.