We report the first isotopic study of an animal host-parasite system. Parasitic, intestinal nematodes,Graphidium strigosumandPassalurus ambiguus, were15N-enriched relative to their host, the European rabbitOryctolagus cuniculus, while parasitic cestodes,Cittataenia denticulataandMosgovoyia pectinata, were15N-depleted, suggesting different trophic relationships. Host embryos were more similar in their δ13C and δ15N values to maternal muscle than were any of the parasites. Coprophagy, the direct recycling of food by the rabbit eating its own faeces, did not lead to isotopic differences between stomach contents and faeces, suggesting that the major point for isotopic discrimination in lagomorph nitrogen metabolism is in the animal rather than in the gut. We conclude that bulk δ13C and δ15N can reveal valuable new information about host-parasite relationships, and these could be explored further at the biochemical level using compound-specific isotopic analyses.