Published by and copyright of Marine Ecology Progress Series. The definitive version of this article is available at: http://www.int-res.com/articles/meps2012/455/m455p287.pdf ; Although crabeater seals Lobodon carcinophaga are among the most abundant consumers of Antarctic krill Euphausia superba, their diet has rarely been studied throughout most of the species??? range. Using 8 13C and 8 15N values in vibrissae from 53 seals, we examined the trophic ecology of crabeater seals from the western Antarctic Peninsula (wAP) in 2001, 2002 and 2007. We observed a wide variability in individual seal mean 8 13C values, which ranged from ???19.8??? to ???24.9??? , whereas mean 8 15N value varied from 5.4??? to 7.9???. We identified a positive significant effect of seal mass on 8 13C values, as well as a significant seasonal effect (higher 8 13C values in austral winter), which likely resulted from changes in the composition of the community of primary producers. 8 15N values for crabeater seals, on the other hand, were affected by year, with individuals in 2002 having higher 8 15N values. The median (with range) contribution of Antarctic krill to the diet of crabeater seals, as estimated using the Bayesian mixing model MixSIR, was 87.9% (81.2 to 94.8%). During 2002, krill biomass in the wAP was at one of its lowest levels during the last 2 decades, coinciding with a slight reduction in the importance of krill for the diet of the seals that year, which reached 84.5% (75.1 to 92.4%). Despite the relative plasticity observed in the diet of crabeater seals, it is unknown to what extent, and at what rate, crabeater seals might be able to switch to a more generalized diet, which might impact their fitness, given the ongoing environmental change along the wAP. ; This research was supported by the National Science Foundation through the Office of Polar Programs (grants ANT- 0440687, 0840375, 0523332, and 0838937), National Undersea Research Program, National Oceanographic Partnership Program through the Office of Naval Research, and the Marine Mammal Commission. L.A.H. was supported by CONICYT-Fulbright (Chile). B.I.M. was supported by a NSF predoctoral fellowship and EPA STAR fellowship.