Fifty-three plant essential oils were tested for their toxicity against eggs and adults of Tetranychus urticae Koch as well as adults of Phytoseiulus persimilis Athias-Henriot, by using a filter paper diffusion bioassay without allowing direct contact. Responses varied according to oil type and dose, and mite species. In a plastic container (4.5 by 9.5 cm) bioassay at 14 × 10⁻³ μl/ml air, caraway seed, citronella java, lemon eucalyptus, pennyroyal, and peppermint oils gave >90% mortality against adult T. urticae, whereas 82 and 81% mortality was observed with sage and spearmint oils, respectively. With the exception of sage oil, the other six essential oils were highly effective against T. urticae eggs at 9.3 × 10⁻³ μl/ml air. Against adult P. persimilis, these six test oils caused >90% mortality at 7.1 × 10⁻³ μl/ml air. Particularly peppermint oil at 4.7 × 10⁻³ μl/ml air was highly toxic. In an acrylic cage (30 by 30 by 40 cm) test, lemon eucalyptus, pennyroyal, peppermint, and spearmint oils were highly effective against adult T. urticae at 1.4 × 10⁻³ μl/ml air. These results indicate that the mode of delivery of these essential oils was largely a result of action in the vapor phase via the respiratory system. The essential oils described herein merit further study as potential fumigants for T. urticae control.