Three trials were performed to test the infectivity of a vaccinal and a pathogenic strain of Babesia bovis for the tick Boophilus microplus. The progeny of engorged female ticks fed on calves inoculated with the pathogenic strain were able to transmit the infection to splenectomised calves (measured by the presence of the parasite in their blood and seroconversion), whereas the progeny of engorged females fed on calves inoculated with the vaccinal strain did not transmit the organisms. These results appear to show that this strain of Babesia bovis would not be transmitted by ticks under natural conditions. This is a considerable advantage since the organisms used in the vaccine would not be able to establish foci in the field although they would be able to confer a long-lasting immunity.