We demonstrate high contrast quantum interference between one-photon and three-photon absorption pathways in an organic solid at room temperature. Illumination of a thin polymer film activated with fluorescing dendrimer chromophores of large three photon absorption cross section with two simultaneous femtosecond pulses at near-IR frequency. and its third harmonic UV frequency 3. results in a spatial interference fringe pattern observable by eye. (C) 2005 Optical Society of America