The effect of crystalline order on the exciton diffusion length (L-D) is shown for the archetypal organic semiconductor 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA; see figure). L-D increases from similar to 6.5 nm in the amorphous limit to similar to 25 nm for single crystals. These changes in diffusion length are connected to changes in the fluorescence quantum yield, which is useful for understanding where crystal-size-dependant diffusion trends are likely to be observed for other organic materials.