Male Juan Fernández fur seals (Arctocephalus philippii) are significantly larger than females at birth and show extreme dimorphism as adults. We investigated morphological differences among male and female pups using a cross-sectional sampling design to evaluate whether pup growth rates were sex-specific during the breeding season. We characterized growth rates using mass, length, and girth and found that length was the least variable measure of body growth (based on the coefficients of variation for the 3 measures of body size). Male pups were heavier on average than female pups on any given day of sampling but did not grow faster than females. No significant differences were noted in the body conditions of male and female pups. These findings suggest that the sexual differences among pups of A. philippii originate before birth and are not accentuated while suckling during the breeding season.