The receptive field and submodality characteristics of individual neurons within the cytoarchitectural and topographic subdivisions of the head and face areas of the postcentral gyrus (SI) were determined with the technique of extracellular recording. Correlation of the single-unit data with the intracortical location of the recording electrode provided a detailed description of the functional organization within each of the several cytoarchitecturally distinct regions contributing to the representation of the head and face in SI. The data indicate that the functional organization of the SI cortex which receives its principal input from trigeminal mechanoreceptors is comparable to the organization within those SI regions which receive their input from the mechanoreceptors of the limbs, trunk, and tail. In each topographic subdivision of the SI cortex 1) a single region in the periphery is represented several times in widely separated locations, each time in a context of different submodalities and peripheral receptive fields; and 2) neurons belonging to the different submodality classes are segregated so that projections from cutaneous afferents terminate mainly in cytoarchitectural area 3 in the adjacent anterior portion of area 1, while projections from the afferents innervating deep tissues terminate mainly in cytoarchitectural area 3a, area 2, and the posterior part of each 1. Although the mechanoreceptor input to SI is segregated according to submodality and the mechanoreceptors from most body regions project to multiple widely separated regions within SI, neurons with receptive fields confined to the ophthalmic division of the trigeminal peripheral innervation field are found within a restricted region of the anterior postcentral gyral crown which is positioned symmetrically about the junction of cytoarchitectural areas 1 and 3. Neurons with receptive fields confined to the maxillary division of the trigeminal innervation field are found within a ring of cortex which a) completely surrounds the representation of the ophthalmic field, and b) includes parts of cytoarchitectural area 2, 1, 3, and 3a. SI neurons with receptive fields restricted to the mandibular division of the trigeminal innervation field occupy the largest portion of the SI face area and form a ring of cortical cell columns which completely surrounds that cortical region which receives its input from the maxillary peripheral innervation field.