A characteristic feature of the rat somatosensory neocortex is a discrete topographic representation of the facial whiskers. Afferent fibers projecting to this vibrissae representation were 'bulk-labeled' by injecting horseradish peroxidase into the white matter. Terminal arbors of specific thalamocortical afferents were characterized by their discretely organized layer IV plexus. The size of the terminal arbors varied with their location and paralleled the regional distribution of different size vibrissae related-clusters, indicating that a single afferent rabor tends to fill a cluster. This interpretation was supported by two additional observations. One, when two fibers terminated within a single cluster their terminal arbors appeared largely coextensive. Two, when fibers were reconstructed from tangential sections, terminal arbors appeared to be concentrated within single clusters. The observations, together with the results of previous studies, indicate that the size of a terminal arbor innervating a vibrissa related cluster is related to the extent of the peripheral innervation of the corresponding vibrissa.