Essential oils of Artemisia absinthium L. and Tanacetum vulgare L. were extracted by three methods, a microwave assisted process (MAP), distillation in water (DW) and direct steam distillation (DSD), and tested for their relative toxicity as contact acaricides to the twospotted spider mite, Tetranychus urticae Koch. All three extracts of A. absinthium and of T. vulgare were lethal to the spider mite but to variable degrees. The LC₅₀ obtained from the DSD oil of A. absinthium was significantly lower (0.04 mg/cm²) than that of the MAP (0.13 mg/cm²) and DW (0.13 mg/cm²) oil of this plant species. DSD and DW extracts of T. vulgare were more toxic (75.6 and 60.4% mite mortality, respectively, at 4% concentration) to the spider mite than the MAP extract (16.7% mite mortality at 4% concentration). Chromatographic analysis indicated differences in composition between the more toxic DSD oil of A. absinthium and the other two extracts of this plant, indicating that a sesquiterpene (C₁₅H₂₄) compound present in the DSD oil and absent in the other two may enhance the toxicity of the DSD oil. Chemical analysis of the T. vulgare extracts indicated that β-thujone is by far the major compound of the oil (>87.6%) and probably contributes significantly to the acaricidal activity of the oil.