LARGE carnivorous animals, the top members of the trophic chain, are rare, and flesh-eating dinosaurs were rarer still. For years the only known giant theropods were Tyrannosaums rex1 and the poorly known Deinocheirus mirificus2, both from the Northern Hemisphere, but many important new dinosaurs have been dis-covered in the Southern Hemisphere during the past decade, con-siderably increasing our knowledge of ancient ecosystems. Here we report a new giant carnivorous dinosaur from the Upper Creta-ceous of northwestern Patagonia (Argentina). This new taxon, Giganotosaums carolinii gen. et. sp. nov., is characterized by aproportionally low skull, a reduced shoulder girdle, and robust vertebrae and hind limbs. It represents a primitive evolutionary iteration of large theropods, and provides an opportunity to exam-ine the Gondwanan dinosaur palaeocommunities and their relation-ships to those from Laurasia. Several characters place G. carolinii within the Tetanurae3, and closer to Neotetanurae4 than to Torvosauroidea4. G. carolinii is the largest theropod ever recorded from the Southern Hemisphere, and is probably the world's biggest predatory dinosaur, having a body 12.5 metres long and an estima-ted weight of 6 to 8 tonnes.