Peripherin is a type III intermediate filament (IF) abundantly expressed in developing neurons, but in the adult, it is primarily found in neurons extending to the peripheral nervous system. It has been suggested that peripherin may play a role in axonal elongation and/or cytoskeletal stabilization during development and regeneration. To further clarify the function of peripherin, we generated and characterized mice with a targeted disruption of the peripherin gene. The peripherin null mice were viable, reproduced normally and did not exhibit overt phenotypes. Microscopic analysis revealed no gross morphological defects in the ventral and dorsal roots, spinal cord, retina and gut, but protein analyses showed increased levels of the type IV IF alpha-internexin in ventral roots of peripherin null mice. Whereas the number and caliber of myelinated motor and sensory axons in the L5 roots remained unchanged in peripherin knockout mice, there was a substantial reduction ( approximately 34%) in the number of L5 unmyelinated sensory fibers that correlated with a decreased binding of the lectin IB4. These results demonstrate a requirement of peripherin for the proper development of a subset of sensory neurons.