Biosynthesis of bacterial flagella involves the coordinated expression of 30 or more genes in several separate operons. We have recently shown that in Bacillus subtilis, the sigma 28 factor is essential for flagellar synthesis, suggesting that transcription of these genes is directly under the control of this alternative sigma factor. In enteric bacteria structural genes for flagellar, chemotaxis, and motility operons appear to be under coordinate control, however, the nature of the regulatory factors has not been determined. Sequence analysis of many such genes has failed to reveal plausible promoter sequences for the predominant bacterial RNA polymerase, and several such genes are not transcribed effectively in vitro by the Escherichia coli sigma 70 RNA polymerase. However, all of the sequenced flagellar, chemotaxis, and motility operons from the enteric bacteria are preceded by DNA sequences highly homologous to B. subtilis sigma 28 promoters. We propose that an alternative sigma factor controls expression of the flagellar regulon in both B. subtilis and in the enteric bacteria.