Six homoflavonoids, ophioglonin (1), ophioglonin 7-O-beta-D-glucopyranoside (2), ophioglonol (3), ophioglonol prenyl ether (4), ophioglonol 4'-O-beta-D-glucopyranoside (5), and isoophioglonin 7-O-beta-D-glucopyranoside (6), together with five known compounds, quercetin, luteolin, kaempferol, 3,5,7,3',4'-pentahydroxy-8-prenylflavone, and quercetin 3-O-methyl ether, were isolated from Ophioglossum petiolatum. Their structures were elucidated by analysis of spectroscopic data and chemical evidence. Compounds 1 and quercetin 3-O-methyl ether showed slight anti-HBV surface antigen activity at 25 microM.