Résumé : Two new furanocembranoid diterpenes, crematofuran [(1R*,11R*,12R*)- 6,19:11,12-bisepoxycembrar-3,6,8(19),15-tetraene] (1) and isocrematofuran [(1R*,3S*,4S*)-3,4:6,19-bisepoxycembra-6,8(19),11,15-tetraene] (2), have been isolated from the Dufour gland secretion of the Brazilian ant Crematogaster brevispinosa rochai. Their structures, including the relative configuration, have been determined by a combination of NMR and molecular mechanics methods. The toxicity of 1 towards other ants is on the same level as that of nicotine. This is the first report of cembranoid diterpenes as defensive compounds in an ant. (C) 2000 Elsevier Science Ltd.