Biofilms are multicellular aggregates of bacteria that are organized by an extracellular matrix. A key determinant of biofilm formation by a variety of bacteria is the proper regulation and assembly of the matrix components. In this issue of Molecular Microbiology, Chai et al. present the surprising finding that the genes encoding the components of the extracellular matrix in Bacillus subtilis are expressed in only a subpopulation of cells. Their results indicate that biofilms contain specialized cell types that are differentiated by transcriptional regulation. The population heterogeneity that they observe is another significant example in a growing list of phenotypes that arise owing to bistable gene expression.