We have used Southern hybridization analysis to characterize the extent of fim homology in recognized type 1 fimbriae mutants of Escherichia coli K12, including strains HB101, P678-54, and VL584. We have found extensive homology in strain HB101, and confirm that P678-54 lacks the majority of fim DNA. Strain VL584 contains a deletion of the entire fim region. We have used a new allelic exchange procedure to generate novel fim deletion derivatives of strains MG1655, MM294, and YMC9. To increase the utility of the new deletion strains we also isolated recA derivatives of each mutant. These strains facilitate the isolation, characterization, and manipulation of cloned fimbriae genes from diverse sources.