Little is known of the biology and importance of the ectomycorrhizal symbiosis in the Dipterocarpaceae. This project was undertaken: 1) to follow dynamics of mycorrhizal infection of dipterocarp seedlings at different sites in the forest, to characterise the major fungal associations involved; 2) to follow mycorrhizal infection of dipterocarp seedlings under laboratory conditions with different inoculum sources; 3) to determine whether dipterocarp ectomycorrhizas function in a manner similar to temperature ectomycorrhizas in the uptake of specific nutrients. Twenty-four different ectomycorrhizal types were described from roots of newly germinated seedlings, two to seven month-old seedlings and wildings of Shorea leprosula (Miq.), and approximately 20 year-old S. acuminata Dyer, S. dasyphylla Foxw., S. leprosula and S. parvifolia Dyer trees. Seventeen types were tentatively identified to family level. Vesicular-arbuscular mycorrhizas were not found. In the forest, some 14 day-old S. leprosula seedlings were already ectomycorrhizal but infection could be absent up to seven months after germination. The results implied that hyphal connections were important in early infection of seedlings in the vicinity of parent trees. Mycorrhizal infection of sequentially sampled two to seven month-old seedlings declined over the sampling period at two sites in Gombak, Selangor and one in Ulu Langat, Selangor. Five to six ectomycorrhizal types were dominant on seedlings at each site and a succession of types was observed on seedling roots. At final harvest, increased plant growth was significantly correlated with ectomycorrhizal infection only at one site in Gombak where infection by 'dominant' types exceeded 30%. Non-mycorrhizal seedlings of S. acuminata, S. leprosula, Hopea helferi and H. odorata were able to grow normally in sterile soil under non-competitive situations. Seedlings were able to form ectomycorrhizas even with inoculum present in grassland soils or with inoculum from different host species in the case of H. helferi. Increased phosphorus uptake by ectomycorrhizal seedlings of S. acuminata, S. leprosula and H. odorata was demonstrated.