The heat shock response in Escherichia coli and related bacteria is primarily mediated by σ32 or its homologue (RpoH protein) specifically required for transcription of heat shock genes encoding molecular chaperones and proteases. Extensive work in E. coli revealed some of the mechanisms controlling the cellular level and activity of σ32 during the heat shock response. Recent isolation of a number of RpoH homologues from γ, β and α proteobacteria provided an opportunity to examine evolutionary conservation and diversity of regulatory mechanisms in these bacteria. We here summarize the present status of this aspect of the stress response not only by comparative sequence analysis but by examining the response of representative RpoH homologues of the γ subgroup to heat shock stress. Current evidence indicates that the basic strategy of enhancing RpoH level as a primary response to heat shock stress is well conserved, but the detailed mechanisms for enhancement of the heat shock σ factor level vary among different species that may reflect diverse ecological niches.