We have derived the nucleotide sequence of a segment of the operator-promoter region of the galactose operon of E. coli, by using a variety of DNA sequencing analyses. We have previously reported the sequence of the 5' terminal portion of gal mRNA [Musso, R. E., de Crombrugghe, B., Pastan, I., Sklar, J., Yot, P. & Weissman, S. (1974) Proc. Natl. Acad. Sci. USA 71, 4940-4944] and of the 59 base pairs preceding the startpoint of gal transcription (J. Sklar, S. Weissman, R. Musso, R. Di Lauro, & B. de Crombrugghe, submitted). In conjunction with those results, the present data provide the sequence of the gal operator-promoter region. This sequence is compared with similar sequences in other promoters and operators. Tentative mechanisms for the regulation of the galactose operon are discussed.