Toxicities of (E)-cinnamaldehyde and (E)-cinnamic acid and their 41 structurally related compounds to adult Dermatophagoides farinae Hughes and Dermatophagoides pteronyssinus Trouessart (Acari: Pyroglyphidae) were examined using fabric-circle contact plus fumigant and vapor-phase mortality bioassays. Results were compared with those of two acaricides, benzylbenzoate and dibutyl phthalate. In contact plus fumigant mortality bioassays, the most toxic compounds were (E)-cinnamaldehyde, methyl (E)-cinnamate, cinnamyl acetate, and hydrocinnamaldehyde against adult D.farinae (17.5-23.3 mg/m2) and D. pteronyssinus (19.0-24.0 mg/m2), based on 24-h 50% lethal concentration (LC50) values. These compounds were significantly more toxic than either benzyl benzoate (LC50, 64.9 and 60.5 mg/m2) or dibutyl phthalate (218.9 and 232.3 mg/m2). The toxicity of allyl cinnamate versus benzyl benzoate was not significantly different. Structure-activity relationship indicates that structural characteristics, such as types of functional groups, carbon skeleton, and saturation, appear to play a role in determining the compound toxicities. In vapor-phase mortality bioassays, these compounds were effective against adult D. farinae in closed, but not in open containers, indicating that their mode of delivery was largely a result of vapor action. The active compounds described merit further study as potential house dust mite control fumigants with contact action in light of global efforts to reduce the level of highly toxic synthetic acaricides in indoor environments.