Bukit Belalong, Brunei, is a small mountain (913 m) of uniform shale lithology with continuous primary rain forest from its base to its summit. Three 0.25 ha plots were established at each of three altitudes (200, 500 and 850 m) to investigate the altitudinal zonation of the soils and the vegetation. One soil profile from each altitude is described and chemical analyses of the soils indicate that they are similar at all altitudes. In each plot all trees ≥10 cm dbh were measured and identified as far as possible. Maximum tree height was greatest at 200 m (60 m) and least at 850 m (33 m). Tree density (number of individuals ha−1) increased with altitude. The forest at 850 m was the most species- and family-rich, but since the ratio of species to individuals did not vary significantly among altitudes, the higher species richness is attributed partly to the larger number of trees sampled. The Dipterocarpaceae was the most important family in terms of basal area at all altitudes, but its proportion of basal area was much smaller at 850 m than at the other altitudes. The Dipterocarpaceae was the most diverse family at 200 m and 500 m, but at 850 m its diversity was exceeded by the Lauraceae, Myrtaceae and Euphorbiaceae and equalled by the Clusiaceae. The forests at 200 m and 500 m are classified as lowland rain forest and that at 850 m is classified as a lower montane forest.