Babesia bovis is a deadly disease of cattle resulting in severe economic losses in the vast regions of the world where it is endemic. If reintroduced into the United States, babesiosis would cause significant mortality in the naïve cattle population. In order to address the risk to U.S. cattle, it is essential to quantify the transovarial transmission efficiency in adult female Boophilus microplus ticks following acquisition feeding on persistently infected cattle. This study tested the hypothesis that infection rates are the same for larval progeny derived from females fed to repletion during persistent or acute infection. Increasing parasite levels during acute infection correlated with an increasing number of females harboring kinetes detectable in hemolymph (r = 0.9). The percent infected larvae ranged from 0 to 20% when derived from females fed to repletion on persistently infected calves and from 4 to 6% when derived from females fed to repletion during acute parasitemia. There was no significant difference in infection rates of larval progeny, implying that the risk associated with the introduction of either persistently infected or acutely infected cattle is equal. Parasite levels ranged from 2.4 x 10(2) to 1.9 x 10(5) in 3-day-fed larvae derived from females fed to repletion on persistently infected cattle. One group of larvae failed to transmit the parasite, suggesting that a threshold level of parasites must be obtained by larval progeny via transovarial transmission in order for larvae to deliver sufficient parasites to infect a naïve host.