We studied the ontogeny of diving behaviour in the Galápagos fur seal (Arctocephalus galapagoensis, Heller 1904). Six-month-old seals spent less than 12% of observation time at sea and were entirely dependent on maternal milk for nutrition. Maximum dive depths for this age group averaged 5.6 m, maximum durations 50 s (N = 12). Modal swim speeds averaged 0.55 m/s; maximum observed swim speeds ranged from 0.9-2 m/s (N = 5). Six-month-old seals swam distances of 2.9 km/24 hrs on average (N = 5). No diel patterns were apparent in their diving behaviour. One-year-old fur seals were the youngest age group to show substantial diving activity, reaching maximum depths of 47.5 m and durations of 2.4 min on average (N = 21). Modal swim speeds averaged 0.9 m/s, maximum speeds ranged from 1.4-2.3 m/s (N = 6). Total distances traveled averaged 15.4 km/trip to sea. Yearlings spent 23.9% of observation time at sea, the majority (> 95%) of that at night. This corresponds to the activity pattern of adult females who dive exclusively at night. Yearlings were the youngest age group to contribute to their own nutrition through independent foraging, but were still dependent on their mothers: no yearlings were successfully weaned in this study. 18-month-old seals were better divers, spending 27.6% of observation time at sea, mostly during the night. Maximum dive depths for this age group averaged 61.1 m; durations 3.1 min (N = 11). Modal swim speeds averaged 1.4 m/s (N = 4), and were comparable to modal speeds of two adult females of 1.2 and 1.6 m/s. Maximum speeds for these juveniles ranged from 2.2-2.7 m/s (N = 4); the two adult females reached 2.8 and 4 m/s respectively. Total travel distances averaged 42 km/trip to sea (N = 4 juveniles). These distances corresponded to those covered by two adult females, 43 and 45 km/trip. Two-year-old fur seals included the youngest that were successfully weaned in this study. They spent 35% of observation time at sea, primarily at night. Maximum dive depths averaged 69.7 m, durations 3.4 min (N = 20). Adult females were the best divers in this study, spending 49.5% of observation time at sea, almost exclusively at night. Maximum dive depths averaged 106.5 m, durations 4.5 min (N = 32). During ontogeny, age was the best predictor of time spent at sea. For animals older than six months, body mass was the best predictor for maximum dive depth as well as maximum and median durations. The physiological maturation process precludes weaning before the age of one year. After that, the diving capacity of young fur seals takes a central role in the weaning process, in conjunction with the temporal patterns of relative prey accessibility.