In NW. Borneo thick series of Tertiary sediments occur which are rich in fossil pollen and spores. The majority of these plant microfossils were derived from the various types of tropical lowland vegetation such as mangrove (Muller, 1964), mixed peat swamp forest and mixed Dipterocarp forest. Some pollen types, however, can be traced to microtherm elements in the montane vegetation. As these cannot have migrated through tropical lowlands, their past distribution is of special interest. It is the purpose of this note to review the stratigraphic occurrence of these montane pollen types and discuss briefly the phyto-geographical significance of the data. The sediments which contain the microfossils can be roughly divided in a near coastal and deltaic facies, characterized by alternating shale and sandstone with subordinate coal beds and a marine facies, consisting mainly of shale with subordinate sandstone and limestone beds. The Tertiary sedimentation in the NW. Borneo Basin is characterized by the alternation of these two main facies, but was locally interrupted during periods of mountain building movements, particularly in late Eocene and late Miocene time. These movements shifted the axis of the depositional basin gradually northwards. The formations of interest are the Oligocene-Miocene Nyalau formation, the Miocene Setap shale, Meligan and Lambir formations, the Miocene-Pliocene Belait formation and a group of younger formations of late Miocene-Pliocene age.