This paper presents the structure and species composition of the primary lowland dipterocarp forest in Berau, East Kalimantan, Indonesia. This study was based on three undisturbed forest plots, 4 ha each, totalling 12 ha, where 93 % of the trees (dbh > or = 10 cm) were identified at the taxa level. The density, basal area and standing volume were on average and respectively 521 trees/ha-1, 31 m2/ha-1 and 383 m3/ha-1. The dipterocarps represented about 25% of the tree population, 50% of the basal area and 60% of the standing volume. In primary forest 538 different taxa were recognised representing a mean of 182 tree species per ha. The structure of the forest of Berau is very similar to that of Sabah or other parts of Nortbern Borneo. However, the main characteristic of this forest is its remarkable richness in dipterocarps, in comparison with the northern parts of Borneo, which exhibits in mean 29 species per ha and 61 species for the 12 ha surveyed. The forests of Sabah are mainly dominated by light-demanding dipterocarp species such as Parasharea spp. and Dryobalanops spp. This could result from important canopy disturbances caused by climatic events like long period of drought or cyclone. The high species richness of the Berau forest may be linked to a longer stability and a relative constancy of the climate in the region. The hypothesis of a possible impact of drought events on the forest dynamics and consequently on species distribution and richness in Borneo is discussed. However, it is stressed that the lack of data for Kalimantan is undoubtedly an handicap for the analysis of phytogeographical variations within the region. In the study area, the first record for Indonesia of the two dipterocarps species Shorea leptoderma Meijer and Shorea symingtonii wood demonstrates that our knowledge of the flora of Kalimantan is still to be improved