The toxicity to adult Sitophilus oryzae (L.) (Coleoptera: Curculionidae) of two cassia oils (Especial and true), four cinnamon oils (technical, #500, bark, and green leaf), and (E)-cinnamaldehyde and its 41 structurally related compounds was examined by residual and vapor-phase toxicity bioassays. Results were compared with those of dichlorvos. In residual bioassays, cassia and cinnamon oils exhibited good insecticidal activity. Based on 48-h LD₅₀ values, the toxicity of allyl cinnmate (0.0003 mg/cm²) was comparable with that of dichlorvos (0.00025 mg/cm²). Potent insecticidal activity also was observed with benzaldehyde, β-caryophyllene, cinnamonitrile, hydrocinnamyl acetate, (E)-4-hydroxycinnamic acid, and α-terpineol (LD₅₀ = 0.003–0.009 mg/cm²). Structure–activity relationships indicate that types of functional groups rather than hydrophobicity or vapor pressure parameters seem to play a role in determining the toxicities to adult S. oryzae. In vapor-phase toxicity tests with weevils, these compounds were more effective in closed containers than in open ones. These results indicate that the effect of the compounds was largely a result of action in the vapor phase. Cassia and cinnamon oils and test compounds described merit further study as potential fumigants for the control of S. oryzae because of their greater activity as a fumigant.