Abstract Saccharomyces cerevisiae has only one putative gene (designated CPL1) for a cysteine protease with a protease domain similar to that of calpain. This gene product shows significant sequence similarity to PalBp, a fungal (Emericella nidulans) calpain-like protease that is responsible for adaptation under alkaline conditions, both in the protease domain and the domain following the protease domain. CPL1 disruptant strains show impaired growth at alkaline pH, but no obvious growth defects under acidic pH conditions. This phenotype is complemented by the wild-type CPL1 gene, and its protease activity is essential for complementation. Disruption of CPL1 also causes reduced sporulation efficiency and promotes the degradation of the transcription factor Rim101p, which is involved in the sporulation pathway and has been shown to accumulate in a C-terminally truncated, active form under alkaline conditions. Furthermore, expression of the C-terminally truncated Rim101p suppressed the alkaline sensitivity associated with CPL1 disruption. These results indicate that a calpain-like cysteine protease, Cpl1p, plays an important role in alkaline adaptation and sporulation processes, via regulation of the turnover and processing of the transcription factor Rim101p.