The diving patterns of 10 adult female California sea lions (Zalophus californianus) were examined during the summer breeding season on San Miguel Island, California, in 1982 and 1983 using time–depth recorders. During 17 feeding trips, representing 40.6 days at sea, animals made over 8900 dives, the deepest of which was estimated at 274 m, while the longest was 9.9 min. The majority of dives, however, were less than 3 min in duration and 80 m in depth. From estimates of body oxygen stores, we predict that dives up to 5.8 min can be supported aerobically. Therefore, cost–benefit considerations based on prey availability and encounter rate may be more important than physiological limits in shaping the foraging patterns of Zalophus. Sea lions were active virtually throughout their time at sea, resting on the surface for only 3% of the average trip. Peak diving frequency occurred during the twilight hours near sunrise and sunset. Dives were frequent, however, during all hours of the day and were typically clustered into bouts that lasted a mean (±SD) of 3.3 ± 1.5 h. We suggest that these bouts represent active feeding on discrete prey patches. During short bouts (<3 h), dive depth was less variable than for dives occurring between bouts. During longer bouts, dive depth changed in a manner consistent with pursuit of vertically migrating prey. During the 1983 El Niño, sea lions compensated for a reduction in food availability by lengthening dive bouts. These seasonal and diel variations in diving patterns suggest that the rate of prey encounter strongly influences the depth and duration of individual dives.