This study presents a species inventory of sap beetles (Coleoptera: Nitidulidae), together with descriptions of their habitat types, in lowland tropical rainforests in the middle reaches of the Baram River Basin, Borneo (Sarawak, Malaysia). Thirty-two nitidulid species belonging to 14 genera from six subfamilies were collected by banana-baited traps. There was no significant difference in species richness, abundance, diversity, or species composition between the primary and secondary forests, with 13 species being shared.