The alae, longitudinal ridges of the lateral cuticle, are the most visible specialization of the Caenorhabditis elegans surface. They are present only in L1 and dauer larvae and in adults. Little is known about the mechanisms through which at the appropriate stages secretion of cuticle components by the seam cells results in the formation of the alae. Here we show that three proteins, each containing a Zona Pellucida domain (ZP), are components of the cuticle necessary for larval alae development: CUT-1 and CUT-5 in dauer larvae and CUT-3 and CUT-5 in L1s. Transcriptional regulation of the corresponding genes contributes to the stage-specific role of these proteins. Larvae with reduced cut-1, cut-3 or cut-5 function not only lack alae but are also larger in diameter due to an increase in the width of the lateral cuticle. We propose a model in which reduction of the body diameter, which occurs in normal L1 and dauer larvae, is the result of a dorso-ventral shrinking of the internal layer of the lateral cuticle and formation of the alae results from the folding of the external layer of the lateral cuticle over the reduced, internal one. Alae of adults appear to form through a different mechanism.