The nucleotide (nt) sequence of the Vibrio cholerae acfD gene (encoding an accessory colonization factor) has been determined. The acfD gene encompasses 254 nt that are predicted to encode an 88-amino-acid (aa) protein. Additionally, an open reading frame of 184 aa, designated orfZ, was detected that overlaps the 3' end of acfD by 45 nt. Computer-assisted homology searches revealed that OrfZ possesses aa sequence similarity to the C terminus of the fliC product of Salmonella muenchen and S. rubislaw. Additionally, OrfZ shares limited aa sequence similarity to a region of the S. typhimurium fliA product. Interestingly, a V. cholerae acfD::TnphoA mutant demonstrates reduced motility and an altered swarming phenotype in semi-solid media when compared to wild-type V. cholerae. The possibility that the reduced motility and altered morphology result from polar effects on orfZ expression is currently being investigated.