An investigation was made into the availability of mycorrhizal inoculum and the response of tree seedlings to mycorrhizal infection in West Malaysian forests. Spores of vesicular arbuscular (VA) mycorrhizal fungi in the soil were reduced by 25% after selective logging and by 75% after heavy logging. VA infection in the roots of plants persisting on, or colonizing, a heavily logged site was reduced by up to 75%. The most probable number (MPN) of VA propagules in sieved soil was up to ten times greater than spore density, but was also greatly reduced by heavy logging. This resulted in reduced infectivity of soil from the heavily logged site, as demonstrated by reduced VA infection of bioassay plants. The infectivity of soil declined following sun drying, but sun-dried soil devoid of vegetation retained some infectivity even after 12 months storage. Overall the data suggest that root and hyphal fragments are more important than spores as inoculum in disturbed forest, and that in undisturbed forest living roots and hyphae are likely to be important sources of infection. In a pot experiment, shoot growth of two test species, Albizia falcataria (L.) Becker and Parkia speciosa Hassk. responded more to VA mycorrhizal infection than to P fertilization over the range 0-6 g triple superphosphate per 8 kg of soil. The response to inoculation with a cocktail of `introduced' VA fungi propagated in pot cultures was greater than the response to inoculation with `indigenous' fungi propagated in pot cultures from roots and soil collected in undisturbed forests. Another test species, Intsia palembanica Miq., also responded better to mycorrhizal infection than to P fertilization, and better to VA mycorrhizal infection than to ectomycorrhizal infection. Intsia palembanica seedlings growing around mature dipterocarps quickly became ectomycorrhizal, suggesting that at least some ectomycorrhizal fungi infect both dipterocarps and Intsia. Shorea leprosula Miq. seedlings growing naturally in the forest had ectomycorrhizas 20 days after germination, i.e. before they had true leaves, and within 7 months supported up to 11 different ectomycorrhizal fungi. However, seedlings isolated from contact with the roots of mature Shorea trees remained uninfected in the field for up to 6 months. This shows the importance of contact with living ectomycorrhizal roots for early infection of dipterocarp seedlings, a point which should be recognized in logging operations and forest regeneration programmes.