Aggregations by 3 species of dolphins (the bottlenose dolphin [Tursiops truncatus], the short-beaked common dolphin [Delphinus delphis], and the long-beaked common dolphin [Delphinus capensis]) and California sea lions (Zalophus californianus) were investigated in Santa Monica Bay, California. Groups were followed and observed during 201 boat-based surveys conducted in 1997–2001 documenting that sea lions were aggregated in 18.6% of the sightings with bottlenose dolphins (150 bottlenose dolphin sightings) and in 45.9% of the sightings with 1 of the 2 species of common dolphins (98 common dolphin sightings). Aggregations of bottlenose dolphins and sea lions were observed in inshore (<500 m from shore) and offshore (>500 m) waters, whereas common dolphins and sea lions were observed only in offshore waters. These aggregations were often recorded feeding near escarpments and submarine canyons, showing a striking preference for these bathymetric features. The results show that sea lions spend a significant amount of time following dolphins, sea lions initiate aggregation and departure from dolphin schools, these aggregations occur more often than is expected by chance, and no aggressive behavior between sea lions and dolphins was ever observed at or near the surface. I argue that sea lions may take advantage of the superior food-locating abilities of dolphins. This paper provides the 1st detailed description of mixed-species aggregations and habitat usage by 3 dolphin species and sea lions.