The Escherichia coli Rsd protein forms 1 : 1 complexes with sigma70 protein, which is the major factor in determining promoter recognition by RNA polymerase. Here we describe measurements of the levels of Rsd, RNA polymerase, sigma70 and the alternative sigma38 factor. Rsd levels are sufficient to sequester a significant proportion of sigma70 and immunoaffinity pull-down experiments show that this occurs in stationary phase but not in exponentially growing cells. Rsd expression is controlled by two promoters, P1 and P2. Experiments with lac fusions show that the P2 promoter is stronger than P1, that P2 is active in all phases of growth, and that this accounts for the high levels of Rsd.