Escherichia coli FN506 containing pap genes that encode two different P fimbriae adherence specificity types were tested for virulence in a murine urinary colonization model. Strains containing adherence genes on either high copy or low copy plasmids were compared. Bacteria that harbored the adherence genes on high copy plasmids colonized mouse kidneys less well than bacteria with the same adherence genes in low copy even though the high copy strains exhibited greater hemagglutination capacity. Bacteria with either type of P fimbriae were able to colonize but pap-2+ bacteria showed increased colonizing capacity when strains containing pap-1 or pap-2 genes on low copy plasmids were compared. Bacteria containing plasmids with both adherence specificities had a similar colonizing capacity as bacteria with either type separately.