The fur coat of adults and subadults of both sexes of the Amsterdam Island fur seal Arcto-cephalus tropicalis, was described from pelts collected at Marion Island (46°54′S, 37°45′E) in the South Indian Ocean. Biopsy plugs 10 mm in diameter were cut from five sites on each pelt (neck, chest, sternum, abdomen and rump). Pelage colour at these sites is described qualitatively and quantitatively. Guardhair depth varied little with age, but was significantly longer (P>0·005) in the cow (x̄= 22·2 mm) than the bull (x̄= 18·3 mm), and slightly longer (not significant) in the subadult female (x̄= 20·0 mm) than the subadult male (x̄= 19·3 mm). Guardhairs were longest on the neck and shortest on the belly (sternum and abdomen). Underfur depth showed a similar pattern, and on average guardhairs were 43·7% longer than the underfur. Pilosebaceous unit density per 10 mm2 was significantly greater (P>0·0005) in the subadult male (x̄= 93·6) than the bull (x̄= 55·5), and slightly greater (not significant) in the subadult female (x̄= 87·4) than the cow (x̄= 84.7), and varied widely between body regions. The mean number of underfur fibres per pilosebaceous unit varied little with age and sex, being slightly greater (P>0·05) in the cow (x̄= 49·9) than the bull (x̄= 45·4) in the case of type A guardhairs, and significantly greater (P> 0·005) in the cow(x̄= 49.2) and subadult female (x̄= 51·1) than the subadult male (x̄= 45·1) with type B guardhairs. The micro-structure and scale pattern of guardhair and underfur fibres are described.