Extracellular single neuron recording methods are used to study the RFs of neurons comprising area 1 cell columns in unanesthetized Macaca fascicularis monkeys. The RF data obtained in approximately radial microelectrode penetrations demonstrate that the RFs of neurons located within the same area 1 cell columns can differ strikingly, and that it is common for neighboring neurons to possess RFs differing greatly in size or configuration. However, the RF variations detected within a typical area 1 cell mini-column (single cell radial column) appear to be substantially less than the variations observed for nearby neurons lying in different minicolumns. The RF data obtained from arrays of penetrations suggest that the skin representation in the forelimb region of area 1 is organized in a discontinuous, step-like fashion: as a mosaic of discrete 600 micron wide radial cell columns--segregates. Although the RFs of neurons of a segregate can vary substantially in size and configuration, they all share in common a single small area on the skin. The boundaries of a segregate can be mapped precisely because, unlike the situation for neurons located within the same segregate, some of the neurons located on opposite sides of a segregate boundary (belonging to different segregates) have non-overlapping RFs. Furthermore, it appears that within any given segregate there is no systematic shift in RF location as the electrode advances through a sequence of minicolumns. Systematic RF shifts occurred only when the electrode traversed the boundary between neighboring segregates.