Proteus mirabilis, associated with complicated urinary tract infection, expresses mannose-resistant/Proteus-like (MR/P) fimbriae. Expression of these surface structures, which mediate haemagglutination and have a demonstrated role in virulence, undergoes phase variation. By DNA sequence analysis, a 252 bp invertible element was found in the intergenic region between mrpI, the putative site-specific recombinase gene, and mrpA, the primary structural subunit gene. The invertible segment is flanked by identical 21 bp inverted repeats and the presumptive half-sites for recombinase binding show homology to those recognized by FimB and FimE encoded by the Escherichia colifim (Type 1 fimbriae) gene cluster. When amplified by the polymerase chain reaction (PCR) from static broth cultures expressing MR/P fimbriae, the switch region was found in both ON and OFF positions. When PCR was used to amplify agar cultures which do not express the fimbriae, the switch region was OFF only. A canonical σ70 promoter inside the invertible element drives the transcription of mrpA when in the ON position; in the OFF position it is directed away from mrpA but does not appear to drive expression of mrpI. The mrpI gene was able to confer inversion of the mrp switch region in trans from both ON to OFF and OFF to ON. To examine the position of the switch in vivo, urine, bladder, and kidneys from mice transurethrally infected with P. mirabilis were used to prepare template DNA for PCR amplification. In the absence of urolithiasis (urease-mediated stone formation), the switch was found 100% in the ON position, a condition never observed following in vitro culture. We conclude that MR/P phase variation is regulated at the transcriptional level by the action of MrpI on an invertible element and that there is strong selective pressure for the expression of MR/P fimbriae in vivo.