By examining iron contents, it is demonstrated that the monogenean Ancyrocephalus mogurndae (Yamaguli, 1940) feeds on the blood of its host, the mandarin fish Siniperca chuatsi (Basilcwsky), The iron content and then the quantity of blood necessary to produce this amount of iron are found different in young and fully-matured worms. Young worms contain higher levels of iron and estimated amount of blood. It is suggested that/1, mogurndae may start to feed on host blood as attached on gills, and the amount of blood ingested by young worms may vary from 0.01 to 1.00 μΙ before reproduction. The difference between young and fully-malurcd worms may be accounted for by the elimination of haematin and change of food composition in matured worms and may also be affected by reproduction. Experimental infections of the monogenean may provide supportive information for explaining the difference, and further studies should also examine the effect of immune components in host blood or mucus on the intestines of the parasite.