We investigated pinniped ecology at sea in Santa Monica Bay, California. Animals were studied during 277 boat-based surveys conducted in 1997-2007 documenting that California sea lion Zalophus californianus was the most observed species (89%, n sightings = 1393), followed by harbor seal (Phoca vitulina richardsi: 8%, n sightings = 131), and northern elephant seal (Mirounga angustirostris: 1%, n sightings = 15). Sea lions, and occasionally harbor seals, were found in aggregations with bottlenose dolphins Tursiops truncatis in 29% of the sightings (n bottlenose dolphin sightings = 205), short-beaked common dolphins Delphinus delphis and long-beaked common dolphins D. capensis in 53% of the sightings (n common dolphins = 155). Sea lions and harbor seals were regularly observed in coastal waters (shore) but also in the entire bay, with both species showing a preference for submarine canyons. Northern elephant seals were only seen in offshore waters and mostly in proximity of the canyons. The three species were frequently observed traveling (50%, n = 728), thermoregulating (14%, n = 205), and feeding (3.2%, n = 47) but rarely socializing (.21%, n = 3). This is the first long-term study on at-sea distribution, occurrence, frequency and behavior of pinnipeds in Santa Monica Bay [Acta Zoologica Sinica 54 (1): 1-11, 2008].