From fruits of Siraitia grosvenori, a Chinese medicinal plant, a new minor glycoside and four known minor cucurbitane glycosides, siamenoside I (sweet), 11-oxo-mogroside V (sweet), and mogrosides IIE and IIIE (both tasteless) were isolated together with mogrosides IV and V (both sweet) previously isolated from this fruit by Arihara et al. Structure of the new testeless glycoside called mogroside III was elucidated as 3-O-β-D-glucopyranosyl- 24-O-β-gentiobiosyl-mogrol. The relative sweetness of siamenoside I to sucrose was estimated ×563, making this the sweetest compound among the cucurbitane-glycosides so far isolated. The structure-taste relationship of cucurbitane-glycosides is also described.