SUMMARY Cattle were inoculated with a Virginia isolate of Anaplasma marginale Theiler and served as an infective source for laboratory-reared Dermacentor andersoni Stiles and D variabilis (Say) nymphs. Following a molt and subsequent feeding of adult ticks on susceptible cows, transstadial transmission of A marginale occurred from cattle with parasitemias ranging from undetectable (in a carrier cow) to a peak of 27% (in an acutely ill cow). Homogenates of gut tissue from unfed, incubated, and feeding adult ticks that were infected as nymphs were all infective; however, the prepatent period varied according to treatment of ticks after they had molted to the adult stage. Prepatent periods of infections in calves resulting from injection of homogenated tissue from unfed adult ticks was 29 and 34 days longer than the prepatent periods that developed in calves exposed to homogenated tissue from ticks that were feeding or incubated at 37 C for 3 days. Transovarial transmission did not occur when F1 larvae, hatched from eggs produced by female ticks known to be infective, fed on susceptible cattle.