The interaction between catalytically active and inactive Trichoderma reesei cellulase components and cotton fibers has been examined by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Cellobiohydrolase I (CBH I), the major component, was rendered catalytically inactive by its treatment with ammonium hexachloropalladate; however the inactive enzyme still had the ability to bind to the cotton fiber SEM and AFM provided evidence suggesting that the catalytic activity of CBH I was required for fiber disruption, AFM allowing resolution of cotton fibers to the microfibril level. However, at high magnification slightly elongated holes were observed throughout the surface of the microfibrillar surface of cotton fibers treated with inactivated CBH I. No disruption of cotton fibers was observed by a palladium-inactivated CBH II/EG II mixture.