Subterranean clover infected by Glomus fasciculatum was transplanted into soil containing propagules of two other species of vesicular-arbuscular mycorrhizal fungi: Acaulospora laevis and Glomus tenue. Root temperatures were maintained at either 15 or 20 °C and the soil was either watered to 80% of field capacity for 3 weeks or left dry before the seedlings were transplanted. Glomus fasciculatum spread well from existing infection except when the soil contained a highly-infective population of A. laevis. Infection by A. laevis was more rapid at 20 than at 15 °C and was greatly enhanced by pre-wetting of soil. The lag phase in the development of infection was greater for G. tenue than it was for A. laevis. High values for the percentage of the root length infected by G. tenue were recorded only in plants in which infection by A. laevis did not develop rapidly. There was no effect of G. fasciculatum in the transplanted seedlings on the level of infection formed by A. laevis; at 20 °C the percentage root length infected by G. tenue was lower when plants had been infected by G. fasciculatum. Irrespective of species, the fungus which infected most rapidly in each case had the competitive advantage, probably due to its prior occupancy of the root.