Two phytotoxic compounds [2,4-dihydroxy-1,4(2H)-benzoxazin-3-one (DIBOA) and 2(3H)-benzoxazolinone (BOA)] were previously isolated and identified in 35-day-old greenhouse-grown rye shoot tissue. Both compounds were also detected by TLC in greenhouse-grown root and fieldgrown shoot tissue. The concentration of DIBOA varied in the tissues, with the greatest quantity detected in greenhouse-grown shoots. DIBOA and BOA were compared with β-phenyllactic acid (PLA) and β-hydroxybutyric acid (HBA) for activity on seed germination and seedling growth and were consistently more toxic than either compound. Dicot species tested, including lettuce (Lactuca sativa L.), tomato (Lycopersicon esculentum Mill.), and redroot pigweed (Amaranthus retroflexus L.), were 30% more sensitive than the monocots tested. Of the two benzoxazinone compounds, DIBOA was most toxic to seedling growth. DIBOA and BOA reduced chlorophyll production inChlamydomonas rheinhardtii Dangeard, by 50% at 7.5 × 10(-5) M and 1.0 × 10(-3) M, respectively. Both DIBOA and BOA inhibited emergence of barnyardgrass (Echinochloa crusgalli L. Beauv.), cress (Lepidium sativum L.), and lettuce when applied to soil, indicating their potential for allelopathic activity.