The general stress response of the Gram-positive soil bacterium Bacillus subtilis is controlled by the sigma B transcription factor. sigma B activity is regulated by the newly discovered partner switching mechanism of signal transduction, which integrates the two different classes of challenges which posttranslationally activate sigma B: environmental stress and energy stress. Our investigation of a possible sigma B homologue in the related soil bacterium B. licheniformis had two goals. First, this study would contribute to understanding the distribution of the sigma B general stress system among Gram-positive bacteria. Second, a phylogenetic comparison of regulatory systems can supplement genetic and biochemical analysis by revealing conserved features that are critical for function. We report here that (1) B. licheniformis cells contain a protein that closely resembles B. subtilis sigma B in size and antigenic properties; (2) the level of this potential sigma B homologue rapidly increases following environmental or energy stress; and (3) the B. licheniformis genome encodes a homologue of the sigB general stress operon, including the sigma B structural gene and seven rsb regulatory genes. Based on these results, B. licheniformis possesses a general stress system likely regulated by two coupled partner switching modules that sense and integrate the two broad classes of activating stress signals.