The claustrum of the cat was studied to determine the extent of its projection to the cerebral cortex. Neurons in the bilateral dorsal claustrum are labeled by retrograde axonal transport of horseradish peroxidase (HRP) from injection sites of different neocortical areas with preponderance to the ipsilateral side. It is revealed that the dorsal claustrum projects to almost entire regions of the bilateral neocortex and that distribution of labeled neurons in the bilateral claustrum has topographical correspondence, rostro-caudal as well as dorso-ventral, with regard to the sites of injections in the neocortex. No neurons in the ventral claustrum are labeled.