In connection with the results obtained by studying the attack on Valonia ventricosa by helicase preparations (Wardrop & Jutte 1968) further investigations were carried out to obtain a better understanding of the morphological factors relating to the degradation of wood fibres by cellulase preparations. To this end, the effect of degradation by cellulase preparations on Scots pine, beech and ash was studied, with both light and electron microscopy. Both unattacked and soft-rotted wood was used, either intact or as macerated fibres and tracheids. Moreover artificial lignification was used on the isolated wood elements and the Valonia ventricosa fragments before treatment with cellulases. The results obtained suggest that the form of soft rot cavities can be explained by the crystalline nature of the cellulose microfibrils.