Background: While Borneo's forests are globally recognised for their diverse vegetation many regions remain uncharacterised. Aims: We examine the tree community in one hectare of lowland (hill) forest near the Malinau River. Methods: We objectively sited a 1-ha plot in primary forest. All stems over 30 cm girth were measured and identified. Results: Stem basal area was typical for Asian rain forests, but the numbers of stems (759) and species (205) were high. The most abundant species were Gluta wallichii, Cleistanthus bakonensis and Lithocarpus cantleyanus, while those contributing most to basal area were Shorea venulosa, Dipterocarpus lowii and Calophyllum lowii. Dipterocarpaceae was the dominant family amongst the largest stems and contributes a third of stand basal area (11.5 m2). Thirty-three secondary species contribute 24% of stems and 16% of the plot's basal area. Conclusions: This is one of the richest hectares of forest trees reported from Indonesia. Species possess broad edaphic preferences and diverse biogeographic affinities. Tree species density likely reflects disturbance history and climatic stability as well as a combination of site factors reflecting the complex local geology, rugged landscape context, and associated edaphic variation. Further botanical explorations in Malinau and other poorly known regions of Borneo are required.