In the natural environment the majority of bacteria live within the confines of a structured social community called a biofilm. Residence offers bacteria multiple advantages over their freeliving cousins that cannot be explained by genetics (1). Many of these benefits are conferred by production of an extracellular matrix, the hallmark feature of biofilms. The biofilm matrix largely consists of proteins, polysaccharides, and DNA. It provides a source of water and nutrients, and confers structural integrity (1–4). Biofilms formed by the Gram-positive bacterium Bacillus subtilis are characterized by a highly wrinkled morphology and a hydrophobic surface. The biofilm matrix is composed of a large exopolysaccharide synthesized by the products of the epsA-O operon, and the TasA/TapA proteins that form fibrous aggregates (5). Assembly of the matrixrequires thesmall, secreted surface-active protein called BslA (formerly YuaB). BslA is found as a discrete layer at the surface of the biofilm despite uniform transcription of the coding region by the entire biofilm population (6–9). It achieves its surface hydrophobicity due to its striking amphiphilic structure, which we recently elucidated by X-ray crystallography (10). The structure of BslA consists of a canonical Ig-like domain, to which is appended a three-stranded “cap” that ishighly hydrophobic in character, rich in leucine residues as well as isoleucine, valine, and alanine (10). In the crystal structure, this cap comprises a surface-exposed hydrophobicpatchof∼1,620Åthat wehavepreviouslyproposed to mediate adsorption to the air/water interface. The large surface-exposed hydrophobic patch exhibited by BslA is a characteristic shared by the unrelated family of fungal