Stable isotopes of nitrogen (δ15N) and carbon (δ13C) were measured for Atlantic salmon Salmo salar and their intestinal cestode, Eubothrium crassum, sharing the same diet. Atlantic salmon muscle tissues were enriched in 15N and depleted in 13C compared to their prey (sprat Sprattus sprattus sprattus) and their intestinal cestode. There was no significant difference in δ15N or δ13C between E. crassum and the sprat. Differences in nutrient uptake and intestine physiology between Atlantic salmon and E. crassum are discussed, as well as how these may give rise to different fractionations of stable isotopes between a host and its parasites. Furthermore, Atlantic salmon contained a significantly higher lipid content than their prey, which may partly explain differences in δ13C values between the host and its cestode. In addition, cestodes inhabiting lipid-rich hosts were also lipid rich. Larger Atlantic salmon were enriched in 15N compared to smaller fish. Cestodes inhabiting large hosts were also enriched in 15N compared to parasites living in smaller hosts. The last two results were explained by larger fish possibly feeding from a higher trophic level, or from larger and older prey, that resulted in both a higher lipid content and an enrichment in 15N.