Tracings of electron micrographs of mycorrhizal roots of onion were measured using an image-analysing computer and the results were used to derive estimates of the volume occupied by the fungal arbuscules, the increase in volume of host cytoplasm after infection and the area of interface between fungus and host. The average lifespan of an arbuscule was deduced from the proportions of living and senescent arbuscules measured in light microscope preparations. These measurements were used to evaluate two hypothetical mechanisms for transfer of phosphorus from fungus to host. It was concluded that digestion of the fungus by the host was not likely to be a significant factor in phosphorus transfer. Transfer of phosphorus across the membranes of living fungus and host appeared to be a more probable mechanism.