Miniproteins provide useful model systems for understanding the principles of protein folding and design. These proteins also serve as useful test cases for theories of protein folding, and their small size and ultrafast folding kinetics put them in a regime of size and time scales that is now becoming accessible to molecular dynamics simulations. Previous estimates have suggested the "speed limit" for folding is on the order of 1 mu s. Here a computationally designed mutant of the 20-residue Trp-cage miniprotein, Trp(2)-cage, is presented. The Trp(2)-cage has greater stability than the parent and folds on the ultrafast time scale of 1 mu s at room temperature, as determined from infrared temperature-jump experiments.