The Salmonella typhimurium fimA gene is controlled by several ancillary fim genes. One of these genes, fimZ, appears to be involved in increasing the expression of fimA. A fimZ mutant of S. typhimurium was constructed by allelic exchange, and this mutant was found to be nonfimbriate. The fimZ mutant demonstrated decreased levels of fimA expression compared with the parental strain when both were grown under conditions favoring fimbrial expression. An examination of the predicted amino acid sequence, deduced from the nucleotide sequence of fimZ, indicated that the FimZ polypeptide possessed a DNA binding motif. Bacterial lysates, derived from strains transformed with recombinant plasmids possessing a fimZ gene, demonstrated DNA binding activity with a fragment containing the fimA promoter. Lysates without a FimZ polypeptide did not exhibit any binding activity. These data are consistent with FimZ being a transcriptional activator of fimA, and FimZ acts by binding to the promoter region.