An outbreak of fatal duck sickness among a resident flock of mixed mallard, Peking white, and mallard-Peking white crossed ducks was investigated and proved to be caused by botulism type C intoxication. The incident was deemed unusual because it occurred on a flowing river in the apparent absence of the usual conditions associated with avian botulism. Furthermore, Clostridium botulinum could not be demonstrated in bottom samples from the shallow water at the site of the outbreak. Although a comprehensive scheme was followed for detection and isolation of C. botulinum type C, and ample evidence of the toxic anaerobe in enrichment cultures was obtained, examination of approximately 125 isolates failed to yield the toxic anaerobe in pure culture. A possible association between this outbreak of avian botulism and an alteration in the aquatic environment occasioned by the building of a high dam, with the attendant rise in water levels and decreased river flow rate, is suggested but can not be definitely proved. Interested workers should be alert to the possibility of botulism in unusual or recently altered environments and the attractive hazard to migratory fowl posed by afflicted resident waterfowl.