The expression of type 1 fimbriae in Escherichia coli is phase dependent, i.e. a cell is either completely fimbriated or bald. This phenomenon is due to the periodic inversion of a specific 300-bp DNA segment containing the promoter for the fimbrial subunit gene, fimA. The phase switch is controlled by the products of two regulatory genes, fimB and fimE, located upstream of fimA. The fimB and fimE proteins direct the phase switch into the 'on' and 'off' position, respectively. The DNA sequence of a 3000-bp region containing the two genes has been determined. The fimB and fimE proteins exhibit strong homology and have most likely originated by duplication of an ancestral gene. They are highly basic implying that they control the phase switch through interaction at the DNA level.