Maneuverability is critical to the performance of fast-swimming marine mammals that use rapid turns to catch prey. Overhead video recordings were analyzed for two sea lions (Zalophus californianus) turning in the horizontal plane. Unpowered turns were executed by body flexion in conjunction with use of the pectoral and pelvic flippers, which were used as control surfaces. A 90 degree bank angle was used in the turns to vertically orient the control surfaces. Turning radius was dependent on body mass and swimming velocity. Relative minimum radii were 9-17% of body length and were equivalent for pinnipeds and cetaceans. However, Zalophus had smaller turning radii at higher speeds than cetaceans. Rate of turn was inversely related to turn radius. The highest turn rate observed in Zalophus was 690 degrees s(-1). Centripetal acceleration measured up to 5.1 g for Zalophus. Comparison with other marine mammals indicates that Zalophus has a morphology that enhances instability, thus providing enhanced turning performance. Enhanced turning performance is necessary for sea lions to forage after highly elusive prey in structurally complex environments.