A novel alkaloid, gaultherialine A (1), along with twenty-seven known compounds were isolated from the whole plants of Gaultheria nummularioides D. Don. The structure of the new alkaloid was elucidated as 7,8-dimethoxy-1-(4-methoxyphenyl)-4,5-dihydro-2H-benzo[e]indole-2-one N-oxide (1) on the basis of spectral evidence.