Fatty acid signature analysis is based on the conservation and unique signatures of fatty acids in the marine food chain and it may be useful in determining the composition of seal diets or in detecting changes in diets. We apply fatty acid signature analysis to milks collected from Antarctic fur seals at South Georgia during the perinatal period (n= 19) and subsequently during early (n=11), mid (n=11), and late (n=8) foraging trip intervals, and demonstrate its use in detecting shifts in prey intake which correspond with changes observed in diving and foraging behavior during a poor food year (austral summer 1990/1991). In lactating otariid females, milk fatty acids secreted during the perinatal fast are derived largely from blubber mobilization and thus likely resemble an integration of dietary fatty acids consumed during the fattening period prior' to parturition, whereas milk fatty acids secreted during foraging trip intervals are derived primarily from immediate dietary intake during the lactation period. To compare groups, we used methods of classification and regression tree (CART) analysis in S-plus, which allows the statistical interpretation of complicated fatty acid patterns containing up to 70 variables per observation. The fatty acid signature of perinatal milks was significantly different (0/19 missclassification rate) from all other milks and late attendance (0/8 missclassification rate) was correctly distinguished from early and mid attendance (1/22 missclassification rate). Our results confirm that adult female fur seals have a distinctly different diet prior to parturition while away from breeding grounds and likely switch to a diet of krill during lactation. Additionally, at least in a poor food year, fur seals do not feed exclusively on krill throughout lactation but rather begin to shift to a diet of teleost fish during late attendance. These findings were consistent with independent 1990/1991 scat data and indicate that fatty acid signature analysis promises to be a valuable tool in trying to understand aspects of foraging ecology in free-ranging pinnipeds.