A new steroid glycoside — alliospiroside B (I) — has been isolated from the collective fruit ofAllium cepa L. On the basis of chemical transformations and with the aid of physicochemical measurements it has been established that compound (I) has the structure of (25S)spirost-5-ene-1β,3β-diol 1-O-[O-α-L-rhamnopyranosyl-(1 → 2)-β-D-galactopyranoside. Compound (I) C39H62O3, mp 200–202°C (from ethanol). [α]D20 −110.9±2° (c 1.01; pyridine) was obtained by extracting the collective fruit ofA. cepa with ethanol folowed by the column chromatographic separation of the combined glycosides on silica gel. The acid hydrolysis of (I) gave (25S)-ruscogenin (II), C27H42O4, mp 189–191°C, [α]D23 −104.1±2° (c 0.98; pyridine). The1H and13C NMR spectra are given for both compounds and the IR spectrum for compound (I).