Avian botulism was responsible for the death of over 6,000 waterfowl at Horicon National Wildlife Refuge in Wisconsin in 1978. The outbreak occurred in early fall on a flooded 250 hectare fallow agricultural area on the northeast end of the refuge. The species most severely affected was the green-winged teal (Anas carolinensis), which made up almost 45% of the total birds found. Carcass pick-up, mouse toxicity tests, and antitoxin injections of waterfowl are discussed.