Modelling fur seal populations requires the accurate assessment of demographic parameters such as age-specific mortality. Owing to the highly variable mortality rates that pups are subject to, mortality of this age class is perhaps the most important factor determining the number of surviving individuals within each cohort. Early pup mortality,sex ratio and median pupping date were determined for sympatric populations of Subantarctic fur seals (Arctocephalus tropicalis) and Antarctic fur seals (A. gazella) at Marion Island, Southern Ocean. Mortality for this species was density dependent, varying from 0.8% at low density sites to 10.9% at high density sites. More accurate methods employed at low-density sites showed a substantially higher pup mortality at three weeks of 1.3–3.4% for Subantarctic fur seals. The same method yielded a mortality estimate at four weeks of age of 1.1–5.1% for Antarctic fur seals. Despite the underestimate inherent in island-wide counts for Subantarctic fur seals, these estimates are still useful for observing temporal and spatial patterns. Sex ratios were at parity for newborns of both species. The sex ratio of eight-week-old Subantarctic fur seal pups was also at parity, which is unusual for fur seal populations. The median pupping dates determined for Subantarctic fur seals and Antarctic fur seals did not differ substantially from previous estimates.