In area 18 of the primate visual cortex, staining for the mitochondrial enzyme cytochrome oxidase reveals 3 types of stripelike subdivisions running perpendicular to the 17/18 border: thick, thin, and pale stripes. In a previous paper (Livingstone and Hubel, 1984), we described the anatomical connections with area 17 of 2 of these 3 subdivisions, but we did not have any conclusive information on the third subdivision, the thick stripes. Here we report that, in the squirrel monkey, the main input to the thick stripes from area 17 arises from layer 4B. Layer 4B receives its input from the magnocellular division of the lateral geniculate body by way of layer 4C alpha; the thick stripes therefore probably belong to the magnocellular subdivision of the visual pathway.