This is the first reported case of septic peritonitis caused by Anaerobiospirillum succiniproducens in a dog. The infection was associated with marked exfoliation of reactive mesothelial cells into the abdominal fluid mimicking neoplasia. The source of the infection was not determined but was presumed to be of gastrointestinal origin as A succiniproducens is part of the normal gastrointestinal flora of dogs. Anaerobiospirillum spp. have been previously reported as causing diarrhea and bacteremia in people, particularly if immunocompromised; however, there were no indicators for a compromised immune system in this dog.