The aim of this study was to evaluate the fumigant and repellent activity of five essential oils (from eucalyptus, geranium, lavender, mint, and orange oil) and seven monoterpenes (eucalyptol, geraniol, limonene, linalool, menthone, linalyl acetate, and menthyl acetate) on first-instar nymphs of the bloodsucking bug Rhodnius prolixus Stahl (vector of Chagas disease in several Latin American countries). Fumigant activity was evaluated by exposing the nymphs to the vapors emitted by 100 microl of essential oil or monoterpene in a closed recipient. The knockdown time 50% (KT50) for eucalyptus essential oil was 215.6 min (seven times less toxic than dichlorvos, a volatile organophosphorus insecticide used as a positive control). The remaining essential oils showed a poor fumigant activity: < 50% of nymphs were knocked down after 540 min of exposure. The KT50 values for monoterpenes, expressed in minutes, were as follows: 117.2 (eucalyptol), 408.7 (linalool), 474.0 (menthone), and 484.2 (limonene). Eucalyptol was 3.5 times less toxic than dichlorvos. No affected nymphs were observed after 540 min of exposure to geraniol, linalyl acetate, or menthyl acetate. Repellency was quantified using a video tracking system. Two concentrations of essential oils or monoterpenes were studied (40 and 400 microg/cm2). Only mint and lavender essential oils produced a light repellent effect at 400 microg/cm2. Geraniol and menthyl acetate produced a repellent effect at both tested concentrations and menthone only elicited an effect at 400 microg/cm2. In all cases, the repellent effect was lesser than that produced by the broad-spectrum insect repellent N,N-diethyl-3-methylbenzamide (DEET).