Abstract.The ability of adultStomoxys calcitrans(L.) (Diptera: Muscidae) to retain viableEhrlichia risticii(Rickettsiaceae), the aetiologic agent of Potomac horse fever (PHF), and mechanically transmit the pathogen from citrated bovine blood artificially infected withE.risticiito susceptible mice was studied. ViableE. risticiiwere found in the digestive tract ofS.calcitrans3h after the flies had engorged to repletion on infected blood; however, noE. risticiiwere detected in flies ≥2 days after feeding. Subsequently, groups ofS.calcitranswere fed for 20 s on infected blood, then fed to repletion on mice 30, 60,130, 180 or 220 min after having feeding interrupted. Mice displayed no clinical signs of PHF and did not produce anti‐E. risticiiantibodies when assayed 30 days afterS. calcitranshad fed. AlthoughS.calcitranswere able to harbour viableE.risticiifor at least 3h, transmission of the disease agent to susceptible mice during interrupted feeding was not demonstrated under these experimental conditi