Substances unfavorable to the growth of guayule plants appear in cultures containing actively growing plants of this species. These substances are organic in nature and can be extracted from nutrient soln. which has been flushed through gravel cultures of guayule plants. The growth-inhibitory extracts were assayed for their toxic activity by application to young guayule seedlings grown in soln. cultures under standard conditions. A potent source of growth-inhibitory substance was found in water in which guayule roots were allowed to soak briefly. From this water 2 compounds inhibitory to the growth of guayule plants were isolated in crystalline form: cinnamic acid, and a 2d unidentified organic acid.