The strength in vivo of 14 promoters was determined in a system which permits the quantitation of RNA synthesis with high accuracy. Up to 75-fold differences in promoter strength were measured and the most efficient signals are promoters from coliphages T7 and T5. Their activity approaches the strength of fully induced promoters of the rRNA operons which may be close to the functional optimum of a single sequence. By contrast, a synthetic 'consensus promoter' belongs to the less efficient signals. Our data show that optimal promoter function can be achieved by alternate structures and strongly suggest that information outside of the 'classical' promoter region contributes to promoter activity.