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Uninoculated dipterocarp seedlings raised in normal field soil in nurseries were always found to have mycorrhizas after a few months. This study set out to determine whether dipterocarp seedlings could continue to grow and develop in the absence of mycorrhizas and also to determine possible sources of mycorrhizal infection of  dipterocarp  seedlings  raised  under  laboratory  conditions  using  Shorea  acuminata  as  a  typical  example. Seedlings were planted in capped or uncapped perspex boxes containing  sterile or non-sterile  field soil and watered  daily  with  sterile  water  or  tap  water.  Seedling  growth  and  development  of  mycorrhizas  were monitored at monthly intervals for up to seven months. Seedlings grown in sterile soil remained uninfected after seven months while infection was found in some of the seedlings grown in normal soil regardless of whether they had been watered with tap water or sterile water. This showed that field soil (i.e. under grass) far from the forest contained suitable inoculum for forest tree seedlings. Tap water and the air were not important sources of infection. However, mycorrhizal  infection was  very  uneven  indicating  that  the  inoculum was  probably  very  unevenly distributed in the soil or that the inoculum density was rather low. Seedlings grown in sterile soil showed better growth  than  those  grown  in  normal  soil  and  infection  of  roots  by  parasitic  fungi  in  the  latter was  also observed. Key words:    Mycorrhizas/Plant pathology/lnfections/Shorea  acuminata/Seedlings.