Transcription of the gene osmE of Escherichia coli is osmotically inducible and regulated by the growth phase. Expression of osmE is directed by a single promoter, osmE (p), which is recognized by Esigma(70) and Esigma(s), two forms of RNA polymerase using, respectively, the sigma factors sigma(70) and sigma(s). Esigma(s) transcribes osmE (p) during entry into stationary phase. Esigma(70) is responsible for osmotic induction of osmE (p) during the exponential growth phase. In a search for proteins that can modulate osmE (p) expression in trans, we performed electrophoretic mobility shift experiments using a DNA fragment carrying osmE (p) and crude extracts from E. coli. One major retarded band was observed in these experiments. The Fis protein is responsible for this retarded band, and binds to several sites upstream and downstream of, and overlapping, the promoter region of osmE. In a fis mutant background, the kinetics of in vivo transcription of osmE (p) during growth demonstrated that Fis is not responsible for the repression of the promoter seen during early exponential phase. In contrast, expression of osmE (p) at elevated osmolarity during the mid-exponential growth phase is increased in the absence of Fis, demonstrating that Fis is able to act as a repressor in vivo at a particular stage of growth.