Mechanical transmission of Anaplasma marginale by horse flies (Tabanidae) is thought to be epidemiologically significant in some areas of the United States. We compared the relative efficiencies of mechanical transmission of Anaplasma marginale by the horse fly, Tabanus fuscicostatus Hine, during acute infection (≈10⁷ to ≈10⁹ infected erythrocytes [IE]/ml blood) with biological transmission by Dermacentor andersoni Stiles in the persistent phase of infection (≈10^2.5 to ≈10⁶ IE/ml). Transmission of A. marginale was not observed when horse flies were partially fed on an acutely infected donor calf and immediately transferred to susceptible calves to complete their blood meal. Ticks that were acquisition fed on the same donor host after it reached the persistent phase of infection successfully transmitted A. marginale when transferred to the same recipient calves that failed to acquire infection after fly feeding. Failure of fly-borne mechanical transmission at a rickettsemia >240-fold higher than that from which ticks transmitted with 100% efficiency shows that tick-borne biological transmission is at least two orders of magnitude more efficient than mechanical transmission by horse flies.