A single subcutaneous immunization with a vaccine used for protecting ranch mink (Mustela vison) against type C botulism reduced morbidity and mortality in mallard (Anas platyrhynchos) and northern pintail (Anas acuta) ducks challenged with approximately 4.5 × 10⁴ and 2.25 × 10⁴ mouse lethal doses (MLD₅₀), respectively, of botulinum toxin at 10 and 15 days post-immunization (pi). There was no significant protection at 5 days pi. Protection persisted in mallards for 90 days pi. To simulate use of vaccine as a part of treatment of sick birds in the field, mallards were exposed to toxin and, when clinical signs were evident, each bird was treated by intraperitoneal injection of type C botulinum antitoxin and one-half of the birds were immunized. Immunization had no significant effect on recovery from intoxication. At 10 days post-treatment, all birds were challenged with toxin. Clinical signs and mortality were significantly less frequent among immunized birds than among non-immunized birds after the second exposure. Immunization might be useful as part of the treatment regimen in botulism outbreaks.