The Mentawir Beds, so named by L. M. R. Rutten, comprise a succession of Neogene sediments outcropping in the Balikpapan Bay area and originally thought to be of Late Miocene age. Subsequently their lower part was correlated with the Lower Balikpapan Layers and classified as Tf2. The type locality in the Sungai Mentawir area visited by Rutten in 1911 yielded a few Larger Foraminifera and Mollusca. A re-examination of the latter and a revision of the overall data now suggest a Preangerian age, Tf3. Also, the age determination, Tf3, of a related fauna from the basal Menkrawit Beds of Mangkalihat Peninsula made in 1941 is reconsidered. No unquestionable new departure emerging, the writer adheres to a Preangerian age as originally based on the examination of both Mollusca and corals, despite conflicting evidence brought forward by Larger Foraminifera investigations.