In order to improve the understanding of food-web interactions in sub-Antarctic freshwater systems, we complemented earlier experimental studies with analyses of differences in stable isotopes (N-15 and C-13) among organisms in two lakes with contrasting productivity. The distribution of the stable isotopes showed that the small copepod Boeckella michaelseni feeds mainly on pelagic POM (particulate organic material), whereas the larger copepod species B. poppei also feeds on benthic algae. Furthermore, the predatory copepod Parabroteas sarsi seems to mainly feed on B. michaelseni, but also on B. poppei and the benthic cladoceran, Alona weineckii. Moreover, stable-isotope data suggest that the diving beetle, Lancetes angusticollis, is not only feeding on B. poppei as indicated from experimental studies, but also on the benthic cladoceran A. weineckii. Although the food webs of the two lakes are very similar, they show considerable differences in the distribution of stable isotopes. We conclude that monitoring, experiments and stable-isotope analysis in combination give a reasonably clear picture of sub-Antarctic freshwater food webs.