The aims of this work were to identify the compounds and to investigate the acaricidal activity of the essential oil of Lippia sidoides Cham. (Verbenaceae) for unengorged larvae and nymphs of Rhipicephalus sanguineus and Amblyomma cajennense (Acari: Ixodidae). The oil was analyzed by gas chromatography and gas chromatography/mass spectrometry. In total, 22 compounds comprising 98.5% of the total peak area were identified. The major constituent of the essential oil was thymol (69.9%). The acaricidal activity against larvae and nymphs was assessed using a modified larval packet test. In all experiments, oils were tested at concentrations of 2.35, 4.70, 9.40 14.10 and 18.80 mg/mL. The mortalities of larvae and nymphs of R. sanguineus were 20.6, 47.8, 73.6, 99.5 and 99.0% and 12.0, 50.0, 76.3, 96.0 and 96.1%, respectively. For larvae and nymphs of A. cajennense the rates of mortality were 41.9, 63.3, 77.8, 82.5 and 100.0% and 0.0, 32.8, 64.8, 71.1 and 94.0%, respectively. The LC 90 values of the L. sidoides oil were 11.56 and 12.97 mg/mL for larvae and nymphs of R. sanguineus and 15.70 and 18.52 mg/mL for larvae and nymphs of A. cajennense, respectively. The essential oil from L. sidoides has acaricidal activity on unengorged larvae and nymphs of R. sanguineus and A. cajennense.