Three antifeedants for larvae of the yellow butterfly, Eurema hecabe mandarina DE L'ORZA, were isolated from Osmunda japonica THUNB. and identified as osmundalin, parasorboside and methyl (3S, 5S)-5-hydroxy-3-(β-D-glucopyranosyloxy)hexanoate. In the course of isolation of the antifeedants. a new glycoside, dihydroisoosmudalin (9), was isolated together with maltol β-D-glucopyranoside, 2-deoxy-L-ribopyranolactone, 5-hydroxymethyl·2-furfural and glycerin. The structure of 9 was elucidated as (4R, 5S)-5-(β-D-glucopyranosyloxy)hexan-4-olide on the basis of chemical and spectroscopic evidence.