The gram-negative organism causing abortion in dogs was examined in parallel with cultures representative of the Brucella species and with Bordetella bronchisepfica. The organism fits into the genus Brucella and most closely resembles B. suis on the basis of its growth characteristics. It is of rough colonial morphology and is agglutinated by antisera prepared against rough Brucella. In mouse toxicity tests, no endotoxic activity could be demonstrated. In contrast to most Brucella cultures, it does not utilize erythritol. Electron microscopy showed a cell wall structure similar to that of other gram-negative organisms. The question of whether the organism should be designated Brucella canis, as proposed by Carmichael and Bruner, or Brucella suis biotype 5 is discussed. The authors favor the designation Brucella canis because the organism lacks the lipopolysaccharide antigen associated with the smooth agglutinogen and endotoxin, and it does not utilize erythritol.