Eucalyptus regnans mycorrhizae vary from simple to coralloid forms, and their surfaces are commonly smooth or conspicuously enmeshed with external mycelium. At least 18 different basidiomycetes may be intimately associated with dense weft-type mycorrhizae; six agarics have been induced to form simple or diffuse mycorrhizae under sterile conditions. Individual mycorrhizae vary in form and colour with both age and the nature of the symbiont. In the field, mycorrhizae are longer-lived than uninfected lateral roots. The earliest stages of infection of new roots appear to occur via root hairs, and intracellular hyphae are produced. This is rapidly followed by the development of either ectendotrophic, ectotrophic or superficial mycorrhizae. Mycorrhizal development is enhanced by high light intensity, diminished by high levels of nitrogenous fertilizer, but apparently unaffected by high levels of phosphate. Experiments suggest that a transfer of carbon-14 takes place from the leaves to the mycorrhizal mantle. Inoculation of E. regnans by Mesophellia arenaria in relatively rich loams of south Gippsland improved the growth of E. regnans seedlings in both field and the glasshouse. The sterilization by gamma-rays of krasnozem topsoil from the forest improved the growth of E. regnans seedlings. Growth of seedlings was further enhanced by the inoculation of such soil with a mycorrhizal fungus (probably inocybe olivaceofulvus).