Crystalline monolayers of fullerene have been self-assembled at the interface between a 1,2,4-trichlorobenzene solution and an Au(111) surface. In situ scanning tunneling microscopy shows the adsorbate arrangement to be structurally equivalent to that found in films grown by sublimation in ultra-high vacuum. This new kind of ordered monolayer might be useful for performing reactions on fullerenes at the interface between their organic solutions and a solid substrate.