Corynebacterium ulcerans was isolated from nares of one asymptomatic dog kept in an animal shelter in the metropolitan area of Rio de Janeiro, Brazil. The RNA polymerase beta subunit-encoding gene was sequenced to confirm the species identity. C. ulcerans strains producing phospholipase D, but not diphtheria toxin, are able to cause severe disease in humans, such as pneumonia and granulomatous nodules in pulmonary tissues. The infection rate varies really widely by region, probably because of the variations in the reported infection rates. Dogs with unapparent C. ulcerans infections may be considered as potentially capable of infecting other animals and humans, including pet owners. Medical and veterinary staff should be aware that asymptomatic animals can carry C. ulcerans and cooperate in eliminating infections and monitoring animals also in the developing countries.