Based on field observations of the effects of the resinous tarweedHemizonia fitchii A. Gray (Asteraceae) on mosquito populations in California, the volatile oil of this plant was investigated for insecticidal activity. Analysis of thé oil by TLC and capillary GC-MS showed the presence of five major constituentś which were identified as the monoterpenoid 1,8-cineole, and the chromenes encecalin, eupatoriochromene (desmethylencecalin), 6-vinyl-7-methoxy-2,2-dimethylchromene, and desmethoxyencecalin. Trace amounts of several volatile fatty acids, alkanes,p-coumarate derivatives, additional chromene derivatives, and numerous mono- and sesquiterpenoids were also detected and identified by GC-MS. Fractionation of the oil by preparative TLC and column chromatography afforded the major chromenes, the identities of which were confirmed by NMR and IR spectral data. The chromenes exhibited weak to moderate toxicity againstCulex pipiens (house mosquito) larvae andOncopeltus fasciatus (large milkweed bug) nymphs. However, no antijuvenile hormone activity was observed for any of the compounds tested against these insect species.