Bioassay-guided fractionation of the chloroform extract from the roots of Angelica sinensis led to isolation and characterization of (Z)-ligustilide using direct-bioautography with Colletotrichum species. The structure of (Z)-ligustilide was confirmed by H-1 and C-13 NMR spectroscopy and GC/MS. (Z)-Ligustilide deterred the biting of two mosquito species more effectively than DEET. Three different A. sinensis accessions and one Angelica archangelica root oil were evauated by GC and GC/MS, and the dominant component in A. sinensis was 61-69% (Z)-ligustilide. Two other prominent compounds in A. sinensis oils were 5.7-9.8% (E)-3-butylidene phthalide and 1.5-2.3% (Z)-3-butylidene phthalide. The main constituents that comprised A. archangelica oil were monoterpene hydrocarbons such as 24.5% alpha-pinene, 13.8% delta-3-carene, 10.1% beta-phellandrene, 8.8% p-cymene, 8.4% limonene, and 6.3% sabinene. Phthalides and monoterpene hydrocarbons were determined to be good systematic markers or chemical fingerprints for A. sinensis and A. archangelica root oils. Chemical fingerprinting by GC/MS of A. sinensis also confirmed the misidentification of one A. archangelica sample sold in the Chinese market.