A pattern of engorgement and dropping of the female cattle tick Boophilus microplus is described. Partly engorged females, which have grown to a length of 4–6 mm (10–30 mg), undergo rapid final engorgement at night to reach a length of 8–11 mm (150–250 mg) and detach from cattle in the early hours of the morning. The minimum size of females which engorge varies with season. Differences in the engorgement and dropping pattern of ticks on cattle observed in summer and in winter, and on cattle held in open yards and in covered stalls, suggest an influence of the external environment, particularly temperature and light, on tick engorgement and detachment. Counting of ticks 4.5–8 mm in length on one day was shown to provide a reliable estimate of the numbers of engorged ticks dropping the following day and has been adopted for the assessment of tick numbers on cattle.