Haemophilus influenzae is a common gram-negative pathogen that initiates infection by colonizing the upper respiratory tract epithelium. In previous work, we reported the isolation of a locus involved in expression of short, thin surface fibrils by H. influenzae type b and presented evidence that surface fibrils promote attachment to human epithelial cells. In the present study, we determined that the fibril locus is composed of one long open reading frame, designated hsf, which encodes a protein (Hsf) with a molecular mass of approximately 240 kDa. The derived amino acid sequence of the hsf product demonstrated 81% similarity and 72% identity to a recently identified nontypeable H. influenzae adhesin referred to as Hia. In experiments with a panel of eight cultured cell lines, the Hsf and Hia proteins were found to confer the same binding specificities, suggesting that hsf and hia are alleles of the same locus. Southern analysis and mutagenesis studies reinforced this conclusion. Further investigation revealed that an hsf homolog is ubiquitous among encapsulated H. influenzae strains and is present in a subset of nontypeable Haemophilus strains as well. We speculate that the hsf gene product plays an important role in the process of respiratory tract colonization by H. influenzae.