The Rukwa rift basin in western Tanzania is a segment of the western branch of the East African rift system. Three major stratigraphic units have been recognized in the basin (from oldest to youngest): Karroo Supergroup, the Red Sandstone, and the Lake Beds. These units have traditionally been assigned the following ages, respectively, based upon lithologic correlations: Permian-Triassic, Jurassic-Cretaceous, and Tertiary. Recently, microfloral assemblages collected from sidewall cores and cuttings from two deep exploration wells drilled in the basin have provided new data on the age of these formations. The biostratigraphic data suggest the formations should be assigned the following ages: Karroo Supergroup, Late Permian; Red Sandstone, Neogene(?) (Miocene-late Pliocene) and the Lake Beds, late Pliocene-Holocene. The new age dates indicate that the post-Karroo to pre-Miocene history of the basin is one of relative quiescence and the modern rift is a relatively young feature. This interpretation is more consistent with the tectonic histories of the other basins of the Western rift.