Benzoxazinoids are common defence compounds of the grasses and are sporadically found in single species of two unrelated orders of the dicots. In the three dicotyledonous species Aphelandra squarrosa, Consolida orientalis and Lamium galeobdolon the main benzoxazinoid aglucon is 2,4-dihydroxy-2H-1,4-benzoxazin-3(4H)-one (DIBOA). While benzoxazinoids in Aphelandra squarrosa are restricted to the root, in Consolida orientalis and Lamium galeobdolon DIBOA is found in all above ground organs of the adult plant in concentrations as high as in the seedling of maize. The initial biosynthetic steps in dicots and monocots seem to be identical. Indole is most probably the first specific intermediate that is oxygenated to indolin-2-one by a cytochrome P450 enzyme. C. orientalis has an active indole-3-glycerolphosphate lyase for indole formation that evolved independently from its orthologous function in maize. The properties and evolution of plant indole-3-glycerolphosphate lyases are discussed.