Nine bibenzyls and 10 stilbenes were synthesized as analogues of batatasin III, a growth inhibitor isolated from dormant yam bulbils, and examined for their plant growth-regulating activities. The bioassays used were the elongation of dark-grown intact rice coleoptiles, auxin-induced elongation of excised oat coleoptiles, and germination of rape and barnyard grass seeds. In the elongation of intact rice coleoptiles, 3,3′-dihydroxy-5-methoxy- (batatasin III), 3,5-dimethoxy-3′-nitro-, 4′-bromo-3-nitro-, 3-amino-3′-chloro-, 3-amino-4′-chloro-bibenzyls and 3-benzyloxy-4′-bromo-5-methoxy-, 3-benzyloxy-3′,4′-dichloro-5-methoxy-stilbenes were inhibitory, and 4′-bromo-3-nitrostilbene was promotive at a concentration of 100 mg/liter. The results obtained by the other bioassays were qualitatively consistent with these findings, although 3-amino-4′- chlorobibenzyl and 4′-bromo-3-nitrostilbene were not tested in all the bioassays. In the seed germination, which was rather tolerant to the test analogues, batatasin III was inactive but 3-benzyloxy-4′-bromo-5-methoxy- and 3-benzyloxy-3′,4′-dichloro- 5-methoxystilbenes were very active. Thus, if substituted properly, bibenzyls and stilbenes are active without hydroxyl and methoxyl group(s) as the functional group.