Seeds of Acanthus mollis contain (4% by dry weight) the glucoside 1 of 2,4-dihydroxy-1,4-benzoxazin-3-one (DIBOA). Enzymatic hydrolysis of this glucoside yields DIBOA which easily degrades to benzoxazolinone (BOA). DIBOA completely inhibits velvetleaf (Abutilon theophrasti) germination at a concentration of 2 mM; BOA is less active. The glucoside has no effect on germination but causes unusual growth of velvetleaf seedlings. This compound is known to occur in the seedlings (but not the seeds) of several grasses (e.g., corn, rye, and wheat). The phytotoxicity of BOA and DIBOA suggests that they might be involved in the allelopathic activity attributed to rye.