Hypochromic microcytic anemia of crucian carp due to infestation with a trematode, Diplozoon nipponicum, was studied. Host fish were obtained from the river Asakawa. Investigations were carried out 4 times during the period from October 1975 to September 1976. A high incidence of infestation with this parasite was noted in all samples. Infection rates ranged between 60 and 80% in small fish, and in larger fish the level reached 100%. The hemoglobin levels of the host fish decreased with increasing numbers of parasites and the relationships between them appeared approximately linear. In anemic fish, all hematological indices decreased. Thus, the anemia was categorized as a hypochromic microcytic anemia. The anemia was characterized by a high percentage of immature erythrocytes. Exposure to a concentration of 1.0 ppm of Trichlorfon (DEP) for 48 hrs proved to be highly effective in eliminating this parasite.