The mechanism of energy transport in organic molecular crystals can be studied by sensitized fluorescence. One can analyze nearly all of the experimental results on sensitized fluorescence so far reported by a hopping model.1In this model, energy is hopping from molecule to molecule with a characteristic time of about 10−13sec (incoherent exciton). Since one does not observe an activation energy for energy migration in pure organic crystals, there is no firm theoretical foundation for the hopping model, and it seems more satisfactory to explain energy transfer by a free exciton (coherent exciton) mechanism.