Aims: The pathogen Bacillus cereus, which is associated with a number of foods including dairy products, was studied for its response to acid stress during the exponential phase. Methods and Results: Bacillus cereus was found to adapt to acid stress (pH 4.6) when pre-exposed to a non-lethal, inducing pH of 6.3 or to inducing concentrations of heat, ethanol, salt or hydrogen peroxide. Cells were found to maintain their internal pH at a higher level than the external acid pH and adapted cells had a higher internal pH than unadapted cells. A constitutive acid-sensitive mutant that was also heat- and ethanol-sensitive was found to be capable of high levels of adaptation despite its lack of induction of proteins induced in the wild type by exposure to moderate pH (6.3) values. Conclusions: A number of proteins were found to be underexpressed in the mutant compared with the wild type at pH 6.3, including some with homology to ribosomal proteins and to the sporulation regulator RapK, while one differentially expressed band contained two proteins, one of which was homologous to the competence regulator CodY. Significance and Impact of the Study: The work has implications for the processing of B. cereus-associated foods by acidification. The linked developmental processes of stationary phase, sporulation and possibly competence appear to be involved in the response to acid stress.