Contrary to a general rule of stable isotope trophic enrichment, parasites show great variation in isotope fractionation from isotope signatures of their host muscle tissues. One of possible mechanisms is that isotopic variation among host tissues may cause apparent deviation from the actual isotope fractionation between a parasite and a given host tissue consumed by the parasite. To test this possibility, we measured δ^15 N and δ ^13C for some fish parasites and a variety of their host tissues to estimate their actual trophic enrichment factors (Δ¹⁵Ν and Δ¹³C). All of trematodes investigated showed positive Δ¹⁵Ν and nearly zero Δ¹³C values. Cestodes and nematodes showed great variations in their Δ¹⁵Ν and Δ¹³C, which range from -3.27 to +3.45, when they were compared to a particular tissue/mucus. Acanthocephalans showed negative Δ¹⁵Ν and Δ¹³C in comparison with host intestinal mucus, and a crustacean had isotope ratios similar to those of host blood. Moreover, there were variations in Δ¹⁵Ν and Δ¹³C among taxa of intestinal parasites. These results suggested that atypical δ ^15N and δ ¹³C enrichment of parasites is not caused by the variation of δ ^15N and δ ¹³C among host tissues, but possibly by metabolic processes unique to parasites.