The experimental vector competence of five laboratory-reared ixodid tick species representing three genera [Amblyomma americanum (L.), Boophilus microplus (Canestrini), D. andersoni Stiles, D. occidentalis Marx, and D. variabilis (Say)] for Babesia equi (Laveran 1901) was evaluated by delayed transfer of male ticks from infected to susceptible equids or by infesting the latter animals with adult ticks previously fed as nymphs on infected equids. After feeding for 5, 6, or 13 d on acquisition hosts, ticks were forcibly removed and held off the host at 26°C, ≈93% RH, and a photoperiod of 14:10 (L:D) h for 6, 12, or 27 d. Intrastadial transmission to susceptible ponies by D. variabilis males, and transstadial transmission to susceptible burros by B. microplus adults, was demonstrated by blood smear and indirect immunofluorescence serology. The data indicated that male D. variabilis and adult B. microplus, tick species that occur on equids in North America and, in the case of the latter tick, also extensively in tropical and subtropical regions of the world, may be competent natural vectors of B. equi