When MATa cells of Saccharomyces cerevisiae have been treated with the mating hormone alpha-factor an increase in chitin synthase zymogen, as well as chitin content in the cell-wall fraction, have been reported. With a DNA probe derived from the cloned CHS1 gene that codes for chitin synthase I [Bulawa, C. E., Slater, M., Cabib, E., Au-Young, J., Sburlati, A., Adair, W. L. and Robbins, P. (1986) Cell 46, 213-225] a Northern analysis was conducted of CHS1-specific transcripts. alpha-Factor-treated MATa cells revealed more than sixfold elevated steady-state levels of CHS1 mRNA as compared to control cells. MAT alpha cells responded the same way when treated with a-factor although induction rate was somewhat smaller. After hormone application a rapid increase in CHS1 mRNA levels could be observed that occurred also in the absence of ongoing protein synthesis. In order to minimize possible side effects of CHS1-coding sequences on expression and mRNA stability a CHS1::SUC2 chimaeric gene was constructed where 730 bp of the CHS1 promoter region (+20 bp of the coding region) were fused in frame to a fragment of the SUC2 coding region. The fusion protein exhibits invertase activity that has been used to monitor CHS1 promoter activity. By analysis of shortened versions of the CHS1 promoter a 94-bp DNA fragment has been identified that confers hormone inducibility to the CHS1 promoter. According to the published sequence of the CHS1 gene, this fragment contains four repeats of a TGAAACA consensus sequence previously identified in the alpha-factor-inducible BAR1 promoter [Kronstad, J. W., Holly, J. A. and MacKay, V. L. (1987) Cell 50, 369-377]. This heptamer may represent the cis-acting element involved in mating-hormone-mediated gene expression in yeast.