The growth of vesicular-arbuscular mycorrhizal (VAM) fungi in pure culture in the absence of living host roots is an important objective. The isolation of nonsporing VAM fungi from a soil which also contained spore-bearing types would be difficult, since the pieces of infected root would probably contain both types and so both would be present in any culture obtained in this way. One of the first problems associated with a project on VAM may be to isolate strains of fungi from field soils and establish them as individual pot cultures in soil mixes which have been sterilized to kill all other mycorrhiza-forming species. Before resting spores of VAM fungi are surface-sterilized, it may be beneficial to store them as a suspension at 2 to 6°C for a few weeks. The use of axenic plants is essential for the investigation of changes in the host brought about by mycorrhizal infection which are not attributable to the presence of any other microorganism.