Extracts from 40 medicinal plant species in 27 families were tested for their nematicidal activity against the pine wood nematode, Bursaphelenchus xylophilus, in 96-well plate bioassays. Responses varied with plant material and concentration. Extracts of Acorus gramineus (Araceae), Asiasarum sieboldi (Aristolochiaceae), Illicium verum (Illicaiceae) and Kaempferia galanga (Zingiberaceae) showed nematicidal activity against male, female and juvenile nematodes of B. xylophilus at 2000 μg ml−1. Among test plant species, Kaempferia galanga showed the most potent nematicidal activity. As K. galanga was the most nematicidally active of the 40 species, the extracts were chromatographically fractionated and two cinnamates, ethyl trans-cinnamate and ethyl p-methoxycinnamate, were discovered to be responsible for much of the activity. The nematicidal activity of ethyl trans-cinnamate and ethyl p-methoxycinnamate was 100% at 60 μg ml−1.