The "in-plane anisotropy" of a triphenylene derivative in Langmuir-Blodgett (L.B.) films has been investigated. Upon heating, L.B.films exhibit a reversible phase transition appearing as an abrupt increase from a low to a high "in-plane anisotropy" corresponding to the formation of the same hexagonal liquid crystal phase observed in the bulk material at almost the same temperature, with the columnar axis aligned along the dipping direction.