The toxicity of 10 plant essential oils to adults of acaricide-susceptible, chlorfenapyr-resistant (CRT-53), fenpropathrin-resistant (FRT-53), pyridaben-resistant (PRT-53), and abamectin-resistant (ART-53) strains of Tetranychus urticae Koch (Acari: Tetranychidae) and to female Neoseiulus californicus McGregor (Acari: Phytoseiidae) was examined using spray or vapor-phase mortality bioassays. In bioassay with the susceptible adults, lemon eucalyptus (19.3 microg/cm3) was the most toxic oil, followed by peppermint, citronella Java, thyme red, caraway seed, clove leaf, and pennyroyal oils (LC50, 20.6-23.7 microg/cm3). The toxicity of these oils was almost identical against adults from either of the susceptible and resistant strains, even though CRT-53, FRT-53, PRT-53, and ART-53 adults exhibited high levels of resistance to chlorfenapyr (resistance ratio [RR], > 9,140), fenpropathrin (RR, 94), pyridaben (RR, > 390), and abamectin (RR, 85), respectively. Against female N. californicus, lemon eucalyptus (LC50, 21.4 microg/cm3) was the most toxic oil, whereas the LC50 values of the other nine oils ranged from 23.2 to 72.6 microg/cm3. N. californicus was 1-2 times more tolerant than T. urticae to the test essential oils. Thus, these essential oils merit further study as potential acaricides for the control of acaricide-resistant T. urticae populations as fumigants.