Body composition is a direct measure of body condition and as such integrates seasonal variation in foraging success and energy expenditure. In phocids, body condition is a critical determinant of both reproductive effort and ability to survive periods of poor foraging success. We determined body composition in 29 crabeater seals, Lobodon carcinophaga, during austral autumn (April–May 2002) and late winter (August–September 2001, 2002) off the Antarctic Peninsula by measuring blubber depth and taking morphometric (length, girth) measurements. In 15 of these seals, we also measured body composition using the labeled-water dilution technique. The two methods produced similar body composition estimates, with an average difference of 7%. There were no differences in body composition between adult males and females or between autumn and late winter. These findings suggest that by autumn, adult seals have replenished their energy stores following reproduction and molt. Similarly, the absence of seasonal variation in adults indicates that seals are successfully foraging throughout winter. In addition to providing insight into seasonal and age-related variation in body composition, this study provides baseline body condition data that can be used to measure impacts of natural or anthropogenic environmental change on one of the large consumers of Antarctic krill.