Constraints on the timing of Karoo and Ferrar continental flood-basalt magmatism in Africa and Antarctica, respectively, are critical to understanding the relationship of the Karoo and Ferrar to mantle plumes, subduction, and the initial breakup of Gondwana. Although recent work has shown that Ferrar magmas were emplaced over a short interval (<1 m.y.), the timing of magmatism within the Karoo and its relationship to the Ferrar have been problematic. New zircon and baddeleyite U-Pb ages on Ferrar (183.6 ± 1.0 Ma) and southern Karoo (183.7 ± 0.6 Ma) dolerites demonstrate that part of Karoo magmatism occurred during the rapid emplacement of Ferrar magmas. A mantle plume is thought to have been important in the genesis of the Karoo province, whereas lithospheric extension, perhaps related to subduction, has been invoked for Ferrar magmatism. The new ages now suggest that Ferrar and southern Karoo magmatism were related to a single mantle thermal anomaly and rifting event. This event may have produced local rift basins and caused rotation of microblocks in west Antarctica several million years before the breakup of east and west Gondwana.