Gene III (gIII) of Lf, a filamentous phage specifically infecting Xanthomonas campestris pv. campestris, was previously shown to encode a virion-associated protein (pIII) required for phage adsorption. In this study, the transcription start site for the gene and the N-terminal sequence of the protein were determined, resulting in the revision of the translation initiation site from the one previously predicted for this gene. For comparative study, the gIII of Xv, a filamentous phage specifically infecting X. campestris pv. vesicatoria, was cloned and sequenced. The deduced amino acid sequences of these two pIIIs exhibit a high degree of identity in their C-terminal halves and possess the structural features typical of the adsorption proteins of filamentous phages: a signal sequence in the N terminus, a long glycine-rich region near the center, and a hydrophobic membrane anchorage domain in the C terminus. The regions between gIII and the upstream gVIII, 128 nucleotides in both phages, are larger than those of other filamentous phages. A hybrid phage of Xv, consisting of the Lf pIII and all the other components derived from Xv, was able to infect X. campestris pv. campestris but not X. campestris pv. vesicatoria, indicating that gIII is the gene specifying host specificity and demonstrating the interchangeability of the pIIIs.