1. A steroid glycoside (M2), which induces avoidance and other reactions in the mollusc Buccinum undatum, has been isolated from extracts of the starfish Marthasterias glacialis by ion-exchange chromatography. 2. The steroid glycoside was homogeneous by t.l.c. and contained glucose, quinovose, fucose and sulphate in the molar proportions 1:2:1:1, in addition to a water-insoluble aglycone. 3. The aglycone was identified as a cholestane derivative containing an unusual Δ24-23-ketone system, two secondary hydroxyl groups and an olefinic double bond, and had the molecular formula C27H42O3. 4. The rates of release of sugars and sulphate suggested that fucose was at the non-reducing end of the oligosaccharide, with glucose glycosidically linked to the steroid. The sulphate group appeared to be linked to the other hydroxyl group of the steroid.