The neurexins are involved in the formation and function of synapses. Each of three genes encodes alpha- and beta-neurexins. Additional diversity (particularly of alpha-neurexins) arises from alternative splicing, resulting in a large number of protein isoforms, the significance of which is currently unclear. We have analysed alpha neurexin expression and alternative splicing during development of the frog Xenopus laevis. Surprisingly, each alpha-neurexin gene is expressed in immature oocytes. During embryonic development, each Xenopus neurexin (nrxn) gene has a distinct temporal expression pattern, with expression being almost exclusively within the neuroepithelium. The spatial expression of nrxnIalpha and nrxnIIalpha is similar in the developing CNS, with staining being observed in the optic cup and in dorsolateral regions of anterior neural tube, but not adjacent to the ventral midline. The pattern of nrxnIIIalpha expression is more restricted, in several domains of the anterior neural tube. In the forebrain, expression was confined to an area in the ventrolateral neural tube; nrxnIIIalpha was also expressed in the hindbrain and spinal cord. By stage 32, a period when synaptogenesis occurs, nrxnIIIalpha is expressed midway along the neural tube's dorso-ventral axis in the hindbrain and anterior spinal cord, at the site of the primary interneuron column. Because of the striking diversity of neurexin isoforms, we analysed alternative splicing of Xenopus transcripts during development and found examples of alternative splice variants of each neurexin. The data demonstrate differential regulation of the alpha neurexins with respect to the gene temporal and spatial expression and alternative splicing.