The Africanized bee in the western hemisphere descended from 26 Tanzanian queen bees (A. m. scutellata) accidentally released by a replacement bee-keeper in 1957 near Rio Claro, São Paulo State in the southeast of Brazil from hives operated by biologist Warwick E. Kerr, who had interbred honey bees from Europe and southern Africa. Hives containing these particular queens were noted to be especially defensive. Kerr was attempting to breed a strain of bees that would be better adapted to tropical conditions (i.e., more productive) than the European bees used in South America and southern North America. The hives from which the bees were released had special excluder grates which were in place to prevent the larger queen bees from getting out but to allow the drones free access to mate with the queen. Unfortunately, following the accidental release, the African queens eventually mated with local drones, and their descendants have since spread throughout the Americas.

http://www.youtube.com/watch?v=v6hxz6p4gUM