Many synthetic insecticides cause immobilization in insect pests after they are exposed. This immobilization or knockdown is an important feature of intoxication that contributes to the abatement of pest insect populations, while preventing vectors of disease from biting and spreading pathogenic organisms to susceptible individuals. We have previously demonstrated that certain plant essential oils rapidly immobilize adult female mosquitoes that have been exposed via topical application. To further characterize this effect, adult female Aedes aegypti were exposed to multiple concentrations of 32 commercially available plant essential oils, and immobilization at 1 h after exposure was recorded. The dose required to produce the 1-h knockdown effect in 50% of the test population (KD 50) was calculated and compared with concentrations of each plant essential oil that caused mortality at 24 h. In the current study, multiple plant essential oils caused high percentage knockdown at 1 h at lower concentrations than concentrations that caused mortality at 24 h. Moreover, delayed mortality was observed in mosquitoes that were exposed to various concentrations of the 2 plant essential oils that produced significant knockdown at 1 h. These observations demonstrate an important characteristic of many plant essential oils and represent a novel means for which these oils may be incorporated into future insecticidal formulations.