Over a 6-day period beginning on 15 October 2008, 93 dead or sick wild waterfowl, including Mallards, Spotbills, and teal species, were found along the shore of a branch stream of the Hangang River, which flows through Seoul, Korea, and were submitted to the National Veterinary Research and Quarantine Service (NVRQS) for diagnosis. Clinically, the affected birds showed flaccid paralysis of the legs and wings and paralysis of the neck. Grossly, no bird showed any lesions, but all had almost empty stomachs. Histopathologic findings included mild lymphocytic hepatitis and mild lymphocytic interstitial nephritis. Clostridium botulinum type C toxin was identified in sera collected from the birds using a mouse bioassay for botulinum toxins; however, no bacteria were isolated from any of the affected birds. In addition, a low-pathogenic avian influenza virus was isolated from two Spotbills, and pesticides such as diazinon and phorate, were detected in seven Mallards. The cause of this outbreak is not clear, but an increase in organic materials from sewage due to drought, increased temperatures, and an increased number of aquatic carcasses resulting from pesticide contamination may have increased the replication of C. botulinum, contributing to the release of botulinum toxins into the waterfowl food chain.