The addition of gyroscopes to tags have made it possible to describe, for the first time, fine-scale movements and underwater orientation of a free-ranging otariid, the California sea lion (Zalophus californianus). Gyroscopes enable the separation of animal accelerations from acceleration due to gravity, and provide direct measurements of rotation speeds. We instrumented four adult females with archival tags containing accelerometers, magnetometers, gyroscopes and pressure sensors. The tags were attached at the midline of the body, approximately above the center of mass, and remained in place for at least one foraging trip to sea of 5-9 days. We applied a matched filter method to separate dynamic animal accelerations from the gravity signal, producing a record that clearly reveals individual swimming strokes. Previous reports of sea lions swimming describe brief symmetrical strokes of their pectoral flippers interspersed with gliding. In contrast our data show alternating asymmetric stroking patterns during the ascent portion of dives deeper than 60 m. Simultaneous with each stroke, gyroscopes recorded an alternating change in angular velocity from clockwise to counter clockwise approximately 30 deg/s in each direction about a rostral-caudal axis. The animals rotated approximately 90-200 deg during the glide interval between strokes, and then rotated in the reverse direction on the next stroke. In addition, the reconstructed tracks show a zig-zag pattern with alternating headings changing approximately 25 degrees . These patterns are not evident either in horizontal swimming portions of dives or the descent portion of dives where the stroking pattern was far more irregular. We speculate that the asymmetrical stroke patterns might either support greater environmental awareness or that this may be an efficient swimming mode.