Phase variation of type 1 fimbriation in Escherichia coli is associated with the inversion of a 314-bp DNA element, positioned proximal to and upstream of fimA, which encodes the major type 1 fimbrial subunit. This DNA switch region contains a promoter that drives transcription of fimA only when the switch is in the ON orientation. Using chromosomal and plasmid-borne lacZ reporter cassettes, we show here how the global regulator H-NS affects the activity of the fimA promoter. In phase-locked reporter cassettes the activity of the fimA promoter was found to be enhanced in a hns-positive background, but only when the switch was in the ON orientation. Also, the number of fimbriae produced by a phase-locked ON strain was significantly higher in a hns-positive background. By means of competitive gel retardation and DNase I protection assays, H-NS binding to DNA segments adjacent to and within the phase switch region was demonstrated.