On the basis of the 'parallel-up' structure of the cellulose crystal, a crystallographic approach to study the mode of action of cellobiohydrolase Cel7A on Valonia cellulose microcrystal has been carried out. After incubation with Cel7A, most of the initially smooth and well defined Valonia microcrystals displayed fibrillation. However, as the hydrolysis reaction was rather heterogeneous, some microcrystals remained superficially intact. Close investigation on such crystals revealed polar morphology: one end was narrowed extremely or pointed. Electron microdiffraction analysis of these crystals evidenced that the narrowing of the microcrystals occurs at their reducing end side. This was also confirmed by the visualization of selective reducing end labeling at the pointed ends of microcrystals. These lines of investigation are the first demonstration that the processivity of Cel7A action against insoluble highly crystalline celluloses is unambiguously toward non-reducing ends from reducing ends.