Experimental transmission of Ehrlichia canis was conducted using laboratory-reared Rhipicephalus sanguineus ticks that were fed as nymphs on dogs infected with low passage field isolates, or by experimental injection of replete nymphs with white blood cells isolated from infected dogs. After molting, 30 naturally infected and 70-150 injected ticks were applied to E.canis negative dogs. The ticks infected by feeding successfully transmitted E.canis to dogs demonstrated by the development of clinical signs typical of Ehrlichiosis (12 of 12 dogs) and by PCR (10 of 12 dogs). E.canis transmission to dogs was also demonstrated with the experimentally injected ticks through analysis of clinical signs (4 of 4 dogs) and PCR (3 of 4 dogs). This study accomplished successful transstadial and horizontal transmission of E.canis using low numbers of ticks infected with E.canis field isolates that were not laboratory cultured. This study also demonstrated for the first time, successful transmission of E.canis by ticks that were experimentally injected with the bacterium as engorged nymphs.