Tolaasin is a lipodepsipeptide (lipo = hydrophobic side chain, depsipeptide is an amino acid ring closed in a particular way) produced by strains of Pseudomonas tolaasii, and it causes Brown Blotch disease in mushrooms. This is a major problem in commercial mushroom growing. The role of tolaasin is not understood. The protective coating of mushrooms is largely built of hydrophobin, an amphiphile that forms very hydrophobic surfaces. Tolaasin is surface active but phase separates and does not appear to self-assemble in solution. It may therefore solubilize or weaken the hydrophobin layer. Tolaasin is detected by the formation of a "white line" when it comes into contact with some other lipodepsipeptides, which are evidently antagonistic to tolaasin but the mechanism is not understood. These two aspects of the interfacial behaviour of tolaasin will be explored.