Serial sections of cell- and fiber-stained and Golgi-impregnated material from adult mice were used to study the cytoarchitectonics, fiber and neuronal architecture of the inferior colliculus. The size of the cells, the pattern of dendritic branching, and the appearance of the neuropil were the features used to delineate the three main regions of the auditory tectum: the central mass of cells or central nucleus, the cortex, and the paracentral nuclei. The central nucleus contains two major cell types: the bipolar cells, which are the most abundant, and the multipolar cells. The dendrites of the bipolar cells are oriented in the same direction and the afferent axons of the lateral lemniscus run along them, contributing to form fibrodendritic strips: the laminae of the central nucleus. The orientation of these laminae differs in the various parts of the central nucleus and delineates four subdivisions. In these four subdivisions, the laminae maintain the same relative position throughout the anteroposterior axis of the central nucleus, but they stop abruptly at the periphery of the nucleus. The cortex surrounds the central nucleus dorsally and caudally. The lamination in four layers concentric to the surface, the increasing gradient of size from the periphery to the deep tissue, the existence of two major types of cells, stellate and pyramidal, permit this structure to be considered as a true cortex. The paracentral nuclei are scattered around the central nucleus. The commissural nucleus is composed of cells with a simple dendritic branching pattern perpendicular or parallel to the fibers of the intercollicular commissure. The dorsomedial and ventrolateral nuclei are characterized by the presence of large multipolar cells. The nucleus of the rostral pole, distinct from the anterior pole of the central nucleus, is composed of small and medium-sized multipolar cells. The lateral nucleus appears as an extension of the dorsal cortex with only two or three layers of cells. The neuronal organization in the central nucleus appears similar in the mouse and in the cat, suggesting an identical processing of auditory information in the two species. Our results seem to establish definitely the cortical nature of the sheet of cells covering the central nucleus.