ABSTRACTThe entire vegetative life cycle of the vesicular-arbuscular mycorrhizal fungusGlomus intraradixwas followed in a simple monoxenic culture system using Ri T DNA-transformed carrot roots and non-transformed tomato roots as plant partners. Fungal development, from the growth of initial germ tubes to the formation of an external mycelium network and spore production, was observed nondestructively using light microscopy.Surface-sterilized spores isolated from pot culture constituted an excellent source of fungal inoculum. These spores germinated readily on the nutrient medium and only a low rate of contamination was recorded. Germ tubes approaching the surface of isolated roots changed their growth pattern and branched profusely in response to root factors. Throughout the growth process it was possible to observe directly the formation of terminal and intercalary secondary spores, numerous hyphal anastomoses and arbuscle-like structures, intracellular vesicles and spores. The extent of internal root colonization byG. intraradixwas relatively limited, but it was sufficient to permit thein vitroformation of hundreds of new spores free of contaminants, which were viable and capable of colonizing host roots.