The roots of the Thai medicinal plant, Tabernaemontana divaricata (L.) R. Br. ex Roem. & Schult., were investigated for their content of acetylcholinesterase inhibitors. Bioassay-guided fractionation using the Ellman colorimetric method led to the isolation of two bisindole alkaloids, 19,20-dihydrotabernamine and 19,20-dihydroervahanine A. The compounds showed higher inhibitory activity on acetylcholinesterase in comparison with galanthamine, a well-known acetylcholinesterase inhibitor. The inhibitory activity of 19,20-dihydroervahanine A was proved to be specific, reversible and competitive. During the separation process, two inactive bisindole alkaloids, conodurine and tabernaelegantine A, were also isolated. The data suggest that the substitutions at the carbons 11', 12' and 16' might affect the acetylcholinesterase inhibitory activity.