Ten African white-backed vultures (Gyps africanus), 3 lappet-faced vultures (Torgos tracheliotus), and 6 marabou storks (Leptoptilos crumeniferus) were routinely treated for gastrointestinal parasitism with fenbendazole administered in the feed at dosages of 47–60 mg/kg for 3 days. Subsequently, all birds became depressed and anorectic. Results of hematologic testing of severely affected birds revealed profound leukopenia. Six white-backed vultures, 1 lappet-faced vulture, and 1 marabou stork died. Gross necropsy findings were minimal other than enlarged livers and spleens. On histologic examination, severe necrotizing enteritis, bacterial hepatitis, and evidence of septicemia were found. The clinical signs and necropsy findings in these birds were consistent with fenbendazole toxicosis. Fenbendazole at published doses can cause immunosuppression and secondary septicemia in some birds. Therefore, benzimidazole anthelmintics should be used with caution in certain avian species, and appropriate, broad-spectrum antibiotic therapy is indicated in suspected cases of benzimidazole toxicosis.