Ectotrophic mycorrhizas have been studied intensively and their value to some tree species is well known. Endotrophic mycorrhizas have received less attention and their potential value to plants has only been fully substantiated within the last 12 years. Responses of the host to mycorrhizal infection seem always to be associated with improved phosphorus nutrition of the plant. The mechanism whereby this occurs is considered. There is evidence that the ultimate limitation on phosphorus uptake by a simple cylindrical root is the diffusion impedance in the soil around it, and that widely spreading hyphae effectively short-circuit this impedance. Some data on hyphal length, and estimated net flux of phosphorus through the hyphae are given, with a discussion of possible mechanisms driving this flux.