Mammoth. Activision's said that "four or five studios" in total are working on the Call of Duty series, with a total headcount being 500 people.
Dave Stohl, Activision's EVP for studios at the company, made the revelation to VentureBeat.
The line-up includes a rebuilt Infinity Ward following the West/Zampella implosion and Sledgehammer Games, both working together on Modern Warfare 3.
There's also Treyarch working on non-Modern Warfare-based titles, Beachhead on Call of Duty: Elite and Demonware handling the services for all of the games.
A full video interview with Stohl is at the link.