Abstract: In order to reveal the reciprocal interactions between Cymbidium hookerianum and mycorrhizal fungi,the authors studied the location and changes of activity of acid phosphatase (AcPase) in the orchid's mycorhiza using the cytochemical methods.The results showed that AcPase was located in the plasmalemma system, cell wall and nucleus.In the course of infection, the invading hyphae were gradually surrounded and digested by lysosomes of root cells.After hyphae becoming cavum or being completely dissolved, those lysosomes were also disappeared finally.In the cell wall, intercellular gap, plasmalemma and plasmodesma, the activity of AcPase changed from thick to slight, and vanished completely at last, whereas the changes in the cells of lysosomes and hyphae were from slight to thick, then became weakened gradually and vanished finally.The invaded vegetal cell occurred a series of changes.The cell wall was distorted and deformed seriously.Organelles such as chloroplasts, mitochondria and many vacuoles were vanished.The nucleus was deformed and also vanished at last.Lots of lysosomes appeared, and encircled and digested the hyphae.The lysosomes became vanished when the hyphae were dissovled completely.The new nucleus and various organelles, for instance mitochondria, chloroplasts and vacuoles, appeared again in the cell.Moreover, the renewed nucleus could be in the stage of mitosis continuously.