A remarkable feature in the development of basidiomycete fruiting bodies is stipe elongation growth that results primarily from manifold cell elongation. Some scientists have suggested that stipe elongation is the result of enzymatic hydrolysis of cell wall polysaccharides, while other scientists have proposed the possibility that stipe elongation results from nonhydrolytic disruption of the hydrogen bonds between cell wall polysaccharides. Here, we show direct evidence for a chitinase-hydrolyzing mechanism of stipe cell wall elongation in the model mushroom Coprinopsis cinerea that is different from the expansin nonhydrolysis mechanism of plant cell wall extension. We presumed that in the growing stipe cell walls, parallel chitin microfibrils are tethered by β-1,6-branched β-1,3-glucans, and that the breaking of the tether by chitinases leads to separation of these microfibrils to increase their spacing for insertion of new synthesized chitin and β-1,3-glucans under turgor pressure in vivo .