Abstract: Alkalinization of the external environment represents a stress situation for Saccharomyces cerevisiae. Adaptation to this circumstance involves the activation of diverse response mechanisms, the components of which are still largely unknown. We show here that mutation of members of the cell integrity Pkc1/Slt2 MAPK module, as well as upstream and downstream elements of the system, confers sensitivity to alkali. Alkalinization resulted in fast and transient activation of the Slt2 MAPK, which depended on the integrity of the kinase module and was largely abolished by sorbitol. Lack of Wsc1, removal of specific extracellular and intracellular domains, or substitution of Tyr(303) in this putative membrane stress sensor rendered cells sensitive to alkali and considerably decreased alkali-induced Slt2 activation. In contrast, constitutive activation of Slt2 by the bck1-20 allele increased pH tolerance in the wsc1 mutant. DNA microarray analysis revealed that several genes encoding cell wall proteins, such as GSC2/FKS2, DFG5, SKT5, and CRH1, were induced, at least in part, by high pH in an Slt2-dependent manner. We observed that dfg5, skt5, and particularly dfg5 skt5 cells were alkali-sensitive. Therefore, our results show that an alkaline environment imposes a stress condition on the yeast cell wall. We propose that the Slt2-mediated MAPK pathway plays an important role in the adaptive response to this insult and that Wsc1 participates as an essential cell-surface pH sensor. Moreover, these results provide a new example of the complexity of the response of budding yeast to the alkalinization of the environment.