Salmonella serovars were examined for the presence of fim gene sequences using specific DNA probes. All strains, regardless of their ability to express surface-associated fimbriae, retain a considerable amount of DNA homologous to the gene probes used. The phenotypically nonfimbriae FIRN and non-FIRN strains of S. typhimurium retain detectable amounts of fim gene sequences and, therefore, may not be genotypically non-fimbriate. The MS adhesin can be expressed by type 2 fimbriate bacteria when they are transformed with discrete regions of the fim gene cluster. However, this conversion to a hemagglutinating phenotype is not associated with a small region of DNA. Therefore, the inability of type 2 fimbria-producing strains of Salmonella to mediate hemagglutination does not appear to be due to a small deletion in a single fim gene.