The acaricidal effect of the essential oil of the flowers of Ageratum houstonianum on Rhipicephalus lunulatus was tested in the Laboratory of Parasitology of the University of Dschang in the Western Highlands zone of Cameroon. Five doses of the essential oil (0.00, 0.016, 0.031, 0.062 and 0.125 µL/cm²⁾ in four replicates were used. Each replicate consisted of 10 ticks in a Petri dish with filter paper on the base of the dish uniformly impregnated with the product. The results of this study indicate that the essential oil of the flowers is toxic to lunulatus ticks. The mortality after six days of exposure in the control group was 10% while the tested doses had exterminated the ticks. The LD₅₀ was 0.06653 µL/cm² at the end of the first day, indicating a potentially high efficiency of this product on this parasite.