In order to find new acetylcholinesterase inhibitors from natural sources which have therapeutic potential in the treatment of Alzheimer's disease, the chloroform: methanol (1 : 1) extracts of five Lycopodium species (L. clavatum L., L. selago L., L. annotinum L., L. alpinum L., and L. complanatum ssp. chamaecyparissus A. (Br.) Doll.), growing in Turkey (Lycopodiaceae), were screened in vitro by the Ellman spectrophotometric method at 1 mg/ml concentration. Among these extracts, only the extract of L. clavatum was found to be active (49.85%). Bioassay-guided fractionation of the chloroform extract of L. clavatum led to the isolation of alpha-onocerin (IC50 = 5.2 muM), a well-known triterpenoid, as the responsible compound for the activity.