Phase variation of type 1 fimbriation in Escherichia coli is associated with the inversion of a 314-bp DNA element. This DNA switch directs transcription of fimA, encoding the major type 1 fimbrial subunit, in the on orientation but not in the off orientation. Inversion of the DNA element requires either FimB (confers both on and off orientations) or FimE (confers off orientation). Here we show, by chromosomally located fimB- and fimE-lacZ cassettes in isogenic strain sets differing only in the hns locus, how the global regulator H-NS affects the expression of type 1 fimbriae. H-NS was found to downregulate fimB and fimE in a temperature-dependent manner which affected the genes inversely at 30 degrees C and 37 degrees C. By gel-retardation assays H-NS binding was demonstrated to the regions containing the fimB promoter and the fimE promoter, respectively. In vitro recombination analysis suggested no direct involvement of H-NS in the inversion of the phase switch. Rather than directly affecting the switching process per se, it appeared that the orientation of this element was affected by the differential temperature modulation of H-NS of the fimB and fimE genes. Taken together the results suggest that H-NS modulates expression of type 1 fimbriae in a way which seems to favor a fimbriate state at the mammalian body temperature.