Longitudinal sections of birch and Scots pine sapwood maintained in microscope chambers under continuous culture conditions were inoculated with Phialophora fastigiata and Phialophora hoffmannii . Hyphal growth resulting in soft rot decay of wood cell walls was studied continuously using time-lapse cinemicrography. Each stage in the infection of wood cell walls was recorded on film and rates of hyphal growth were determined. Hyphal growth in the chamber was linear but about five times faster than growth within wood cell walls. The formation of soft rot cavities within wood cell walls is oscillatory and determined by successive phases of apical growth of fine hyphae. Chains of soft rot cavities within wood cell walls are formed by the activities of lignolytic enzymes released along the hyphal surface. The chemical composition of the cell wall of the wood species investigated affects the rate of cavity widening.