An isolate of Fusariumroseum "Graminearum" obtained from overwintered oats in Alaska produced toxic cultures when grown on rice. The toxic principle called fusarochromanone isolated from these cultures reproduced the signs of tibial dyschondroplasia in poultry and also reduced hatchability of fertile eggs. Spectroscopic data (nmr, ir, and ms) indicated the mycotoxin to be a chromanone derivative. The chromanone ring structure was also confirmed by single crystal X-ray.