Members of the collapsin/semaphorin family play an important role in creating the complex pattern of neuronal connectivity. Inhibition of growth cone motility by chick collapsin is mediated by the intraneuronal protein CRMP-62. We have now isolated four rat sequences that are highly related to chick CRMP-62. All four genes are expressed exclusively in the nervous system and primarily during development. Rat CRMP-2/TOAD-64 is most closely related to chick CRMP-62 and is the most widely expressed CRMP within the nervous system. Rat CRMP-1 and CRMP-4/rUlip are expressed during discrete periods of neuronal development and are not found in the adult nervous system. Rat CRMP-3 has a distinct distribution, being expressed transiently in developing spinal cord and selectively in the postnatal cerebellum. The differential expression of these genes suggests that CRMPs may transduce signals from different semaphorins and that semaphorins may regulate the plasticity of the adult nervous system.