Confocal laser scanning microscopy was used for in vivo observations of autofluorescent fungal structures of Glomus mosseae in intact, living rye-grass roots. Clear images of hyphae and collapsed arbuscules could be obtained without staining or sectioning roots. As rye-grass roots are highly transparent, with only a few cell layers above the vascular cylinder, they provide an excellent in vivo system for microscopical confocal laser scanning studies of arbuscular mycorrhizal structures.