Onion and Coprosma plants were grown in a range of soils mostly containing very little available phosphate. Very large increases of shoot dry weight (up to eighteen-fold with Coprosma and nineteen-fold with onion) were obtained by adding phosphate. Similar increases (up to fifteen-fold with Coprosma and twelve-fold with onion) were obtained by mycorrhizal inoculation. The response to phosphate equalled or slightly exceeded the response to mycorrhiza, except in two soils rich in phosphate in which there was no response to either, and in one soil in which the mycorrhizal onion and Coprosma plants were twice as heavy as those given phosphate. The greatly increased growth from either treatment was associated with a large increase in the uptake of phosphorus. The probability of such growth responses, but not their size, was usually predictable in soils containing very little phosphorus soluble in CaCl2. In all soils with inoculum mycorrhizal infection was extensive. The sources of phosphorus available to mycorrhizal roots but not to non-mycorrhizal roots differed in the various soils. In some inhibitory factors might make vesicular-arbuscular mycorrhiza less effective in promoting the growth of the host plants.