residence offers bacteria multiple advantages over their freeliving cousins that cannot be explained by genetics ( 1) . many of these benfits are conferred by production of an extracellular matrix, the hallmark iealure of biofilm. the biofilm matrix largely consists of proteins, polysaccharides, and DNA . it provides a source of water and nutrients, and confers structural in tegrity ( 1-4 ) . biofilm formed by the gram-positive bacterium bacillus subtillis are characterized by a highly wrinkled more phology and a hydrophobic surface. the biofilm matrix is comeposed 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 matrix requires the small,secreted surface-active protine called BslA ( formerly YuaB ). BalA is found as a discrete layer at the surface of the biofilm despitc uniform transcription of the coding region by the entirc biofilm population (6-9) it achieves its surface hydrophobicity due to its striking amphiphillic structure, which we recently elucidate by X-ray crystallography (10) .the structure of BslA consist of a canonical Ig-like domain, to wich is character, rich in leucine residues as isoleucine, valine, and alanine (10). in the crystal structure, this cap comprise a surface-exposed hydrophobic patch of 1,620A that we have previously proposed to mediate adsorption to the air/water.