Bacteria can survive harsh conditions when growing in complex communities of cells known as biofilms. The matrix of the biofilm presents a scaffold where cells are attached to each other and to the surface. The biofilm matrix is also a protective barrier that confers tolerance against various antimicrobial agents. In this issue of Molecular Microbiology, Kobayashi and Iwano (2012) show that the liquid permeability of Bacillus subtilis biofilms is determined by a small secreted protein, i.e. BslA (formerly called YuaB). BslA is important for the proper development of biofilms, but unlike exopolysaccharide and TasA, is not directly involved in cell cluster formation, and is synthesized following the production of exopolysaccharide and amyloid fibres. The amphiphilic BslA protein forms a polymer in vitro and localizes in vivo to the surface of the biofilm. The microstructures of the biofilm wrinkles are reduced in the bslA mutant strain and the liquid repellency of the biofilm surface is diminished. Exogenously added BslA 42-181 protein complements the bslA mutation and restores not only water repellency, but also the formation of aerial structures. This study demonstrates that amphiphilic proteins have an important role in liquid repellency of biofilms and it suggests that these polymers contribute to antimicrobial resistance.