Cellulase reaction product was localized cytochemically at the ultrastructural level in the cell wall of disc cells, the secretory cavity and in the subcuticular wall of glands in Cannabis. Cellulase reaction product was evident in the less dense region of the disc cell wall prior to secretory cavity formation. Reactivity in this region was associated with separation of an outer zone, forming the subcuticular wall, from the inner wall zone adjacent to the plasma membrane of the disc cells. Reaction product was associated with the disc cell wall and fibrillar matrix extending from it into the secretory cavity. Reactivity remained evident over the subcuticular wall throughout enlargement of the secretory cavity. Reaction product also was present over fibrillar matrix in the secretory cavity associated with both the inner wall and the subcuticular wall. The distribution of cellulase reaction product supports an interpretation that cellulase is involved in formation of the secretory cavity and subsequent redistribution of wall products to form the subcuticular wall during development of the secretory cavity.