Oral treatment with fumagillin is effective for controlling various microsporean and myxosporean infections in fish. We tested a synthetic analog of fumagillin, TNP-470 (Takeda Chemical Industries), for its efficacy against 2 microsporean pathogens of salmon: Loma salmonae and Nucleospora salmonis. Chinook salmon Oncorhynchus tshawytscha were experimentally infected with either L. salmonae (per os) or N. salmonis (intraperitoneal, i.p., injection) and held in fresh water at 15 degrees C. Fish were then divided into 3 replicate groups: untreated or treated orally at 1.0 mg or at 0.1 mg drug kg-1 fish d-1. With L. salmonae, the high dose fish had 0.32 xenomas mm-2 of gill tissue compared to controls at 24.5 xenomas per mm2. With N. salmonis infections, untreated fish exhibited 100% infection, showed prominent clinical signs (e.g. renal swelling, anaemia), and high mortality. In contrast, fish treated at 1.0 mg kg-1 showed no clinical signs, and 16% of those treated at 0.1 mg kg-1 showed only mild gross pathological changes. With the treated groups, over 50% of the fish exhibited extremely light infections, even with high dose treatments, but no mortalities were attributed to N. salmonis infections. Uninfected fish treated at 1.0 mg drug kg-1 fish d-1 for 5 wk appeared clinically normal and showed no reduction in growth. However, about half of these fish exhibited atrophy of the renal interstitial hematopoietic tissue.