A study on the acaricidal properties of foam soap containing the essential oil of Chenopodium ambrosioides leaves was carried out on Rhipicephalus lunulatus. Four doses (0.03, 0.06, 0.09, and 0.12 µL of essential oil per gram of soap) and a control (soap without essential oil) with four replications for each treatment were used for in vitro trial. Each replication consisted of 10 ticks in a Petri dish with filter paper impregnated with the foam soap on the bottom. Following in vitro trials, three doses (0.06, 0.09, and 0.12 µL/g) and the control in two replications were selected for in vivo test based on mortality rate recorded from the in vitro trial. Each replication was made up of 10 goats naturally infested with ticks. Results show that soap containing essential oil is toxic to R. lunulatus. The in vivo mortality rate in the control on day 8 was 22.69% whereas the highest dose (0.12 µL/g) killed 96.29% of the ticks on day 8. The LD50 of the foam soap containing essential oil was 0.037 and 0.059 µL/g on day 2 in the laboratory and on the farm, respectively. This indicates the potentially high efficiency of this medicated soap on this parasite.