The food pathogen Bacillus cereus is likely to encounter acidic environments (i) in food when organic acids are added for preservation purposes, and (ii) during the stomachal transit of aliments. In order to characterise the acid stress response of B. cereus ATCC14579, cells were grown in chemostat at different pH values (pH(o) from 9.0 to 5.5) and different growth rates (micro from 0.1 to 0.8 h(-1)), and were submitted to acid shock at pH 4.0. Cells grown at low pH(o) were adapted to acid media and induced a significant acid tolerance response (ATR). The ATR induced was modulated by both pH(o) and micro, and the micro effect was more marked at pH(o) 5.5. Intracellular pH (pH(i)) was affected by both pH(o) and micro. At a pH(o) above 6, the pH(i) decreased with the decrease of pH(o) and the increase of micro. At pH(o) 5.5, pH(i) was higher compared to pH(o) 6.0, suggesting that mechanisms of pH(i) homeostasis were induced. The acid survival of B. cereus required protein neo-synthesis and the capacity of cells to maintain their pH(i) and DeltapH (pH(i) - pH(o)). Haemolysin BL and non-haemolytic enterotoxin production were both influenced by pH(o) and micro.