The inferior colliculus of the marmoset was studied in sections prepared by Nissl and Golgi techniques. As in other primates and subprimates it can be divided into a central nucleus (CN) and a cortex, further divisible into dorsal cortical and lateral zones. Afferent fibres enter CN from the lateral lemniscus and fan out from posteroventrolateral to anterodorsomedial, forming a set of laminae along which neuronal somata and dendrites tend to be aligned. Neurons are commonly bitufted or bipolar, and sometimes multipolar, with dendritic arbors oriented along the laminae. Other axons cut across the laminae and are probably efferent fibres destined for the brachium of the inferior colliculus en route to higher levels. The cortex is composed of a variety of bipolar and multipolar neurons and is not laminated. Injection of 2-deoxyglucose while exposing the marmoset to auditory stimulation demonstrates that tones at 4 and 30 kHz cause labelling along the laminae near, respectively, the dorsolateral and ventromedial limits of CN.