Experiments were performed to determine at what phase or phases of its parasitic development Boophilus microplus becomes infected with Babesia bigemina. Problems due to Boophilus microplus being a one-host tick were overcome by artificially transferring ticks from one host to a second just prior to a moult. In several experiments, a babesiacide was used to eliminate parasites so that the exposure of ticks to infection could be restricted to one phase of their development. Further information was obtained by correlating the presence or absence of patent parasitaemia in the bovine with the presence or absence of infection in ticks which fed on it.

It was found that ticks were susceptible to infection for approximately the last day of their parasitic life. Infection at other times, that is, during larval, nymphal and the major part of the adult stages was not usual. Four of five attempts to infect larvae and all five attempts to infect nymphs were negative.

Infection of ticks always resulted when a patent parasitaemia coincided with the last day of parasitic life. Ticks were infected in all thirteen instances when these conditions existed but in only seven of eighteen instances when the animal was a carrier but was not showing parasitaemia.

Stage-to-stage transmission did not occur, although suitable conditions for this existed on nine occasions.