Escherichia coli isolates from patients with bacteriuria of pregnancy were compared by PCR with isolates from patients with community-acquired cystitis for the presence of established virulence determinants. The strains from patients with bacteriuria of pregnancy were less likely to carry genes for P-family, S-family, and F1C adhesins, cytotoxic necrotizing factor 1, and aerobactin, but virtually all of the strains carried the genes for type 1 fimbriae. Standard mannose-sensitive agglutination of yeast cells showed that only 15 of 42 bacteriuria strains (36%) expressed type 1 fimbriae compared with 32 of 42 strains from community-acquired symptomatic infections (76%) (P < 0.01). This difference was confirmed by analysis of all isolates for an allele of the type 1 fimbrial regulatory region (fim switch), which negates type 1 fimbrial expression by preventing the fim switch from being inverted to the on phase. This allele, fimS49, was found in 8 of 47 bacteriuria strains from pregnant women (17.0%) compared with 2 of 60 strains isolated from patients with cystitis (3.3%) (P < 0.05). Determination of the phase switch orientation in vivo by analysis of freshly collected infected urine from patients with bacteriuria showed that the fim switch was detectable in the off orientation in 17 of 23 urine samples analyzed (74%). These data indicate that type 1 fimbriae are not necessary to maintain the majority of E. coli bacteriurias in pregnant women since there appears to be selection against their expression in this particular group. This is in contrast to the considered role of this adhesin in community-acquired symptomatic infections. The lack of type 1 fimbria expression is likely to contribute to the asymptomatic nature of bacteriuria in pregnant women, although approximately one-third of the bacteriuria isolates do possess key virulence determinants. If left untreated, this subset of isolates pose the greatest threat to the health of the mother and unborn child.