The number of neurofilaments and microtubules present in nerve fibers was determined for sciatic nerves from adult mice and from rats of three different ages. More microtubules than neurofilaments were found in nonmyelinated fibers; the ratio of tubules/filaments was reversed in myelinated fibers and was found to change with axon caliber independent of the presence of a myelin sheath. A series of regression analyses indicated that axon caliber correlates best with the sum of the number of neurofilaments and microtubules per fiber. This correlation was only slightly better than that for neurofilaments alone. Axon caliber also correlated better with the filament‐tubular material than with the thickness of the myelin sheath. The results were similar for both rats and mice, and age differences were not apparent in the samples of nerves analyzed.