Crystalline 9-cyanoanthracene undergoes photodimerization to give the trans dimer which is unexpected on the basis of the topochemical preformation theory. The possibility that the reaction occurs at defects is investigated; and the nature of the structural imperfections are described, as are also the types of product nuclei and their modes of growth. Interference-contrast and fluorescence microscopy have been employed for the examination of cleaved and partially dimerized faces of the monomer. It is shown that there is an active slip plane (221), and consideration of feasible dislocation reactions, particularly those involving unit strength dislocations which have a component of the Burgers vector in , reveals that, within stacking-fault regions (bounded by partial dislocations), the monomer molecules are in trans registry. It is suggested that molecules in such stacking faults act as traps for the excitation energy, and that reaction occurs at these sites.