Anaplasma marginale is a tick-borne intra-erythrocytic rickettsial pathogen of cattle which is also mechanically transmitted by biting flies. During acute infection rickettsemia may reach as high as 10⁹ infected erythrocytes/ml of blood (i.e./ml). Surviving animals develop a life long persistent rickettsemia which cycles between 10² and 10⁷ i.e./ml, a level below the threshold for detection on stained blood smears. Although the relative importance of tick-borne vs. fly-borne transmission has not been studied quantitatively, it has been suggested that biological transmission is important during persistent infection, whereas mechanical transmission is more important during acute infection. We have used quantitative PCR to demonstrate that biological transmission by ticks is at least 2 orders of magnitude more efficient than mechanical transmission by stable flies. Stable flies were partially fed on an acutely infected calf (5.8 x 10⁸ i.e./ml) and immediately transferred to susceptible calves. Transmission was attempted with three flies and with 30 flies for each of two calves; none of the 4 stable fly transmission calves acquired infection. After the original acquisition host became persistently infected (1.84 x 10⁶ i.e./ml) the experiment was repeated using ticks as biological vectors. The same 4 calves that failed to acquire infection after stable fly feeding, did become infected after transmission feedings with three or 30 ticks that had been acquisition-fed on the persistently infected host. The rickettsemia at the time of stable fly acquisition feeding was more than 300 times higher than the rickettsemia at the time of tick feeding, demonstrating that biological transmission is at least 300 times more efficient than mechanical transmission.