delta-C-13, delta-N-15, and C/N were measured for each of 247 muscle tissue samples from 12 bird, 4 seal, and 4 fish species collected in the Weddell Sea primarily during March 1986. delta-C-13 values ranged from -31.3 to -22.0 parts per thousand and, in the case of fish and seal samples, varied inversely with C/N. This implied that lipid concentration significantly influenced these vertebrate delta-C-13 measurements. No such relationship was found between C/N and delta-N-15, where the latter values ranged from +4.4 to +11.2 parts per thousand. with considerable overlap among many of the taxonomic groups measured. Notable exceptions to this were found in the Wilson's storm-petrel and the Kerguelan petrel, among which elevated delta-N-15 values of some individuals probably reflected feeding outside of the Weddell Sea. Significant feeding on vertebrate biomass was indicated by the N-15 enrichment of the southern giant fulmar and several snow petrel individuals. Some diet separation among crabeater, Antarctic fur, leopard, and Ross seals was also evident. Nevertheless, the overlap in delta-N-15 among most Weddell Sea vertebrates, coupled with an observed range of vertebrate values that was considerably narrower than that of their potential prey, supports the hypothesis that many of these higher consumers share a small number of common food resources and trophic levels.