Abstract:A cytochemical study has been made to examine the activity of acid β-glycerophosphatase in the mycorrhizal cells of the seedling of Gastrodia elata BI. using thin sectioning technique in which sections were embedded in glycol mathacrylate (GMA). After the seedling was invaded by the hyphae of Mycena osmundicola Lange, two different kinds of infected cells were formed in its root cortex.the outer 1–2 cell layers namely the hyphae-containing cells (or host cells) contained many coiled hyphae pelotons； the inner comparativly large cell layer or fungus-digesting cells contained a few straight hyphae. Localization of acid phosphatase in hyphae-containing cells showed that only a few senescent hyphae retained the enzyme activity and the plant cells did not release hydrolytic enzyme. So it is considered that the hyphal lysis in hyphae-containing cell may be due to autolysis. In contrast, higher acid phosphatase activity was visualized in many vesicles and small vacuoles of the fungus-digesting cells. When a hypha entered a fungus-digesting cell through a hyphae-containing cell, a number of enzyme granules (i. e, enzymecontaining vesicles) gathered around it. Later on the enzyme granules expanded gradually and became small enzyme vacuoles of 1.6–2.0 μm in diameter. Still later the small enzyme vacuoles fused with each other to form a large vacuole in which a part of an invading hypha was enclosed and gradually digested by hydrolytic enzymes. Finally,the digesting vacuole changed into a residual body containing some metabolic waste. The above results suggest that fungus-digesting cells can actively release hydrolytic enzymes by lysosomal vesicles to digest the invading hyphae, but such function is not present in the hyphae-containing cells,the role of which may be attributed to attracting and controling the invading hyphae.