Abstract: The mycorrhizal association is a wellknown mutual relationship between two organisms, a fungus and the root of a vascular plant. A number of mycorrhizal types have been recognized based on the fungal group involved and by the morphological features, resulting in the complex interaction between the symbionts. Various microscopic methods have been used to characterize morphological, anatomical and molecular features of orchid mycorrhizae. Fluorescence microscopy allows optical sectioning of images obtained from different reconstructions which are useful for studying complex fungal structures within the host. Recently, Laser Scanning Confocal Microscopy has stimulated research in the development of new fluorochromes and dyes used routinely for light microscopy. Many dyes have been used routinely for light microscopy. We report here the use of acridine orange for fluorescence microscopy, which is an effective method for a rapid screening of cellular details to study the complex fungal structures of orchid mycorrhizae. Spathoglottis plicata Blume, an ornamental orchid commonly cultivated in several parts of the world, was selected for this study. Its underground bulbs produce several roots that have mycorrhizal association. The plants were maintained in pots with natural inoculum of the mycorrhizal fungus Eupholorhiza repens. The mycorrhizal roots were collected, washed thoroughly in water, cut into small pieces, fixed in formalin–acetic acid–alcohol, dehydrated, embedded in paraffin and sectioned by microtome. Slides with sections were heated gently on a water bath and stained with 0.05% acridine orange (Sigma Chemical Co) for 10 min. Slides were then rinsed with distilled water and allowed to dry. Stained tissues were viewed and photographed directly using Nikon microscope with a filter B-2A excitation and emission at 450 nm In addition, fresh hand sections were also used. Infection and peloton formation were noticed in the cortical cells and it was more in the outer cortical cells. Free hand section also showed a peloton of the mycorrhizal fungus appeared as spherical balls (Figures 1 b, c and 2 c). Individual hyphae forming the peloton could be easily differentiated (Figure 1) from completely filled up cells where recognition of individual hyphae was gradually lost, indicating its slow lysis and digestion (Figures 1 b and 2 c) which supports our earlier findings. Entry of the fungus into the cortex is always through the passage cells of the exodermis and no instance of the presence of fungal mycelium in the thick-walled cells of the exodermis was noticed (Figure 2 a, c). In fact, the exodermal passage cells do have a control on the entry of fungus into the cortex, since they are the only living cells of the exodermis whose cell walls do not have lignin and suberin to the extent that the other exodermal cells have. Like the rhizodermal and hypodermal cells, even in passage cells of the exodermis the fungal hypae do not settle down to form coils, indicating that these cells are meant only as a channel for hyphal entry into the cortex and do not have the appropriate physiological conditions necessary for coil formation and hyphal degradation, although they are more active metabolically. Entry into passage cells is through the outer tangential walls of these cells. From these cells, the hypha spreads into the cortex intracellularly, branches repeatedly and some of these branches get into the cortical parenchyma cells, forming pelotons. More often, the SCIENTIFIC CORRESP