The 16S rRNA gene was amplified, cloned, and sequenced from the blood of two dogs that were experimentally infected with the etiologic agent of canine granulocytic ehrlichiosis. The 16S rRNA sequence was found to be unique when it was compared with the sequences of other members of the genus Ehrlichia. The most closely related species were Ehrlichia canis (98.0% related) and the human ehrlichiosis agent (Ehrlichia chaffeensis) (98.1% related); all other species in the genus were found to be phylogenetically much more distant. Our results, coupled with previous serologic data, provide conclusive evidence that the canine granulocytic ehrlichiosis agent is a new species of the genus Ehrlichia that is related to, but is distinct from, E. canis and all other members of the genus. We propose the name Ehrlichia ewingii sp. nov.; the Stillwater strain is the type strain.