The effect of diffusion on long-range energy transfer by dipole-dipole interaction1,2from electronically excited donor molecules D* to (unexcited) acceptor molecules A may be important in solutions3–7and in molecular crystals7–11. In solutions usually diffusion of matter occurs (the molecules perform a random walk) whereas in molecular crystals an excitation (exciton) migrates. Exciton migration can often be described in terms of an effective exciton diffusion tensor which may be highly anisotropic12–17. Because of the restricted space only the time dependence of the donor fluorescence after δ-excitation will be given. The extension to general excitation forms and to the acceptor flouorescence is a straightforward matter18,19.