Castanospermine, 1,6,7,8-tetrahydroxyoctahydroindolizine, has been shown to be a potent dicot phytotoxin. The effective concentration to inhibit root length elongation of dicot roots by 50% is 300 ppb, while the effective concentration against monocot roots is 200 ppm, i.e., 10(3) times less effective. In contrast swainsonine, 1,2,8-trihydroxyoctahydroindolizidine, is ineffective as an inhibitor of root length elongation.