The potential of fumagillin dicyclohexylamine salt to treat and prevent intestinal giant-cystic disease in Israel carp, Cyprinus carpio nudus, was monitored in field experimental studies. In experiment 1 (therapeutic), most fish were already naturally infected with more advanced stage of Thelohanellus kitauei. Fumagillin was administered to fish (mean body weight of 830 g) for a period of one month at a dose of 10.62 mg in the first group and 5.3 mg in the second group per fish per day. In experiment 2 (prophylactic), most fish also were already naturally infected with an early developmental stage of the protozoa and fish (average body weight of 484 g) were administered fumagillin for 45 days at a dose of 3.95 mg per fish per day. In both experiments, the cumulative mortalities of fish and the extrusion rates of the polar filaments of the spores were significantly decreased in a dose-independent fashion. In experiment 2 no dead fish were observed. No adverse side effects of the drug were observed among fish from any dosage group. In experiment 2, an oval or dot-like concave lesion of most cysts developed at the 7th day and the vegetative form was never observed at the 17th day postmedication and the cysts were grossly reduced in size as compared with the control group, beginning at the 24th day until the end of the study. In contrast, it was scarcely effective to the cysts in experiment 1. Taking the seasonal development of the protozoa into consideration, the above results revealed that oral administration of furnagillin at 3.95 mg/500 g body weight/day for a month is the optimal dose for the treatment and prevention of thelohanellosis caused by T. kitauei among C. carpio nudus.