A deeper mechanistic understanding of the saccharification of cellulosic biomass could enhance the efficiency of biofuels development. We report here the real-time visualization of crystalline cellulose degradation by individual cellulase enzymes through use of an advanced version of high-speed atomic force microscopy. Trichoderma reesei cellobiohydrolase I (Tr Cel7A) molecules were observed to slide unidirectionally along the crystalline cellulose surface but at one point exhibited collective halting analogous to a traffic jam. Changing the crystalline polymorphic form of cellulose by means of an ammonia treatment increased the apparent number of accessible lanes on the crystalline surface and consequently the number of moving cellulase molecules. Treatment of this bulky crystalline cellulose simultaneously or separately with T. reesei cellobiohydrolase II (TrCel6A) resulted in a remarkable increase in the proportion of mobile enzyme molecules on the surface. Cellulose was completely degraded by the synergistic action between the two enzymes.