The neurons of origin of the great cerebral commissures of the marmoset monkey were identified by horseradish peroxidase histochemistry and their distribution was studied. Six adult marmosets were used. Three were normal: the others were subjected to section of the corpus callosum (CC), sparing the anterior commissure (AC). All six were injected with horseradish peroxidase throughout one cerebral hemisphere. The three normals provide information on the origins of both the CC and AC, whereas the three callosotomized monkeys allow study of the origins of the AC alone. All CC and AC neurons in the marmoset are pyramidal cells. Except for layer I, all cortical layers possess commissural cells; their laminar organization varies according to cortical area. There exists a progression in predominance from supra- to infragranular commissural neurons proceeding from temporal through occipital to parietal and finally to frontal cortex. Major acallosal zones are found in the primary visual cortex and the fore- and hindlimb representations of the somatosensory cortex. Correlations between commissural neuron distribution and cytoarchitectonic areas are not always obvious. Commissural neurons were not organized in columnar fashion.