The effects of site conditions on variation in the floristic composition, physiognomy and hollow decay of the Mixed Dipterocarp Forest of Sarawak are investigated. Studies were concentrated on two series of pre-existing plots. The results from the set of larger plots show that there is considerable association between the distribution of floristically defined forest types and site conditions. The results from the smaller plots show that there issome association of site conditions with the distribution of individual species, and also with intraspecific variation in hollow decay. Site effects on the basal areas of individual species, and the basal area and the severity of hollow decay in the forest as a whole are less marked. Interpretation of the results is complicated by the clustered distributions of both series of plots. The relative importance of site properties as determinants of forest floristic variation depends partly on the range of rock, soil and forest types sampled. Over the full range, properties closely related to soil parent material appear to predominate. The results do not indicate any major immediate modifications to current procedures in forest inventory, soil survey or intersectorial land allocation, but they do emphasise the value of interdisciplinary exchanges of information. If regeneration in the forest is to be assisted by planting, the results should be useful in indicating probably viable species/site combinations.