The major mortality of Boophilus microplus on the bovine host occurred during the 24 hours following infestation. Dead and living ticks continued to drop from the host throughout the parasitic cycle. The living ticks dropping from the host were smaller than those developing normally. Most ticks fell from the host at night. The pattern of tick loss, expressed as a percentage of the total ticks lost over the period of study, was essentially the same for both resistant and susceptible animals, while differing in average total numbers dropping, more larvae dropping from the resistant hosts.