Diatom-rich layers were encountered in offshore Angola (block 3) in sediments of the upper Malembo Formation (upper Miocene) drilled by Elf Aquitaine in the Pacassa and Veado wells. The diatom assemblages are composed of a mixture of marine and freshwater taxa. The marine diatoms are mostly littoral; true oceanic forms are rare. Freshwater diatoms, which are often very abundant in the samples, are likely to have been transported by the Congo River and are contemporaneous and/or older than the marine assemblages within which they deposited. They offer a very interesting insight on what the freshwater diatom flora was in western Africa during the Late Miocene. DSDP Site 362 (Walvis ridge) was studied as a reference section (upper Miocene through Quaternary) for this area. Our results suggest that the age of the diatom-rich interval in block 3 wells is late Tortonian, ranges between 8.8 and 7.4 Ma, and corresponds to a period of high-latitude cooling and increased coastal upwelling along the Benguela Margin.