The Saccharomyces cerevisiae SHC1 gene encodes a protein with a high homology to Chs4p, a positive regulator of chitin synthase III (CSIII) during vegetative growth. SHC1 is not expressed during vegetative growth but is strongly induced during sporulation as a mid-late gene. shc1/shc1 mutants do not show any defect in the total rate of sporulation and meiosis occurs normally. However, shc1/shc1 ascospores be-come highly permeable to DAPI, much more sensitive to glusulase treatment, and have very low levels of chitosan in their cell walls. All these observations indicate that Shc1p is required for proper maturation of the ascospore through its participation in the synthesis of the chitosan layer. Lack of SHC1 during sporulation can be partially compensated by over-expression of the CHS4 gene. During vegetative growth, SHC1 has no apparent function but, when ectopically overexpressed, it can substitute Chs4p as an activator of the CSIII activity; however, Shc1p fails to localize it properly, as Chs4p does. In conclusion, S. cerevisiae contains two functionally redundant genes in the control of CSIII activity: CHS4, whose function is restricted to vegetative growth because Chs4p is rapidly degraded during sporulation, and SHC1, whose function in cell wall ascospore assembly is transcriptionally restricted to the sporulation process.