[en] The spreading behavior of a liquid on the surface of a solid substrate is greatly changed by the presence of a molecular layer of organic material (a surfactant) on the liquid surface. In this work, we studied the spreading of water covered by a monolayer of valinomycin on a vertical glass slide, using an apparatus for Langmuir--Blodgett film deposition. The rate of spreading strongly depends on the surfactant concentration, and the spreading front is highly unstable: it bifurcates while spreading, forming tree-like patterns. copyright 1995 American Institute of Physics