The essential oil from aerial parts of Ruta chalepensis L. (Sapindales: Rutaceae) was obtained by hydrodistillation, and its chemical profile was identified using gas chromatography and gas chromatography-mass spectrometry. Compounds, 2-undecanone (43.2%), 2-nonanone (27.9%), and 2-nonyl acetate (10.6%) were the major constituents of the oil. Biting deterrent activity of R. chalepensis essential oil at 10 and 50 mu g/cm(2), 2-undecanone at 8.5 mu g/cm(2), 2-nonanone at 9 mu g/cm(2), and 2-nonyl acetate at 9.3 mu g/cm(2) was similar to DEET (N,N-diethyl-meta-toluamide) at 4.8 mu g/cm(2), against Aedes aegypti L. Biting deterrent activity of R. chalepensis oil at 50 mu g/cm(2) against Anopheles quadrimaculatus Say was statistically similar to DEET at 4.8 mu g/cm(2), whereas the activity was lower in the other compounds tested. In cloth patch assay, R. chalepensis essential oil was effective at 187 mu g/cm(2), whereas 2-undecanone was effective at 108.9 mu g/cm2 against Ae. aegypti. In larval bioassays, 2-undecanone showed similar toxicity whereas toxicity of R. chalepensis essential oil and 2-nonanone was higher at 24-h posttreatment at the LD50 in An. quadrimaculatus than Ae. aegypti. This study revealed that R. chalepensis essential oil and its major compounds were active biting deterrents against Ae. aegypti at higher application rates whereas only the essential oil showed activity similar to DEET against An. quadrimaculatus. 2-undecanone was the most active compound in in vivo repellency bioassay against Ae. aegypti. Chemical composition of R. chalepensis essential oil varies because of plant production and harvest practices, and the activity level of the essential oil may depend on the source of the sample.