The relationship between northern fur seal pup weight and post-weaning survival is examined. Males that survived at least two years after weaning had been significantly heavier than the mean for their cohort as pups. Among a small sample of females no significant relationship was found. Cohort survival from weaning to age two was not significantly correlated with mean pup weight. Pup weight may influence post-weaning survival less than other factors such as gender and variability of the environment. To the degree that it determines survival, pup weight may reflect the reproductive success or fitness of parents. Weight differences between tagged and untagged pups explain, in part, differences observed in their survival.