Aims: To investigate the effect of pH on the viability and the acid tolerance response (ATR) of bifidobacteria. Methods and Results: The impact of low pH on the viability of five species of bifidobacteria was examined under conditions of strict anaerobiosis. Although differences in the ability to resist the lethal effects of low pH were apparent among the species, cell viability could be improved by the provision of fermentable substrate during an acidic pH stress or through the use of stationary phase cells. While a stationary phase ATR was found to occur in two species of bifidobacteria, there was no adaptive response in exponential phase cells. Proteomic analysis of exponential phase Bifidobacterium longum subjected to a mild acid pre-exposure (pH 4 center dot 5, 2 h) prior to an acid challenge revealed a substantial loss in the total number of cellular proteins. In contrast, proteomic analysis of stationary phase cells revealed an increased abundance of proteins associated with the general stress response as well as the beta-subunit of the F0F1-ATPase, known to be important in bifidobacteria acid tolerance. Conclusion: Neither Bif. longum or Bifidobacterium breve possesses an inducible exponential phase ATR. Significance and Impact of the Study: These findings provide further insights into the impact of pH on the viability of bifidobacteria and may partially explain the loss in viability associated with their storage in acid foods.