The expression of the sigB gene of Bacillus subtilis was analysed in response to a mild acid shock. This gene is subject to sigmaB-dependent regulation. It has been found that the expression of sigB is induced as part of the acid-tolerant response. In that respect sigB is similar to the previously described gene gsiB which is also a member of the sigmaB regulon. Through this induction, the sigmaB regulon provides protection against acid shock. Besides its protective role against acid shock, no other general function could be directly associated with the sigmaB regulon. An acidification of the cytoplasmic environment induces synthesis of general stress proteins in B. subtilis.