Hesperozygis ringens (Benth.) Epling (Lamiaceae) is a strongly aromatic plant employed popularly for its antiparasitic properties. The leaves afforded 4% of essential oil constituted mainly by pulegone (86%). Laboratory tests were carried out to determine the toxicity of the essential oil species on engorged females and larvae of the cattle tick Riphicephalus (Boophilus) microplus using the adult immersion test (AIT) and the larval immersion test (LIT). It was observed that the essential oil at the concentration of 50 mu L/mL and 25 mu L/mL inhibited the egg laying significatively in relation to the controls and the eggs from these treated females were affected by the oil: the hatching was inhibited in 95% and 30%, respectively. In the LIT it was verified that the LC(99.9), LC(50) and LC(1) was 0.541 mu L/mL, 0.260 mu L/mL and 0.015 mu L/mL, respectively. Pulegone, isolated from the oil, showed similar effect on the females and on the larvae, indicating that it is responsible for the oil activity. (C) 2009 Elsevier Ltd. All rights reserved.