A cytotoxic glycoside macrolide, callipeltoside A, has been isolated from the marine lithistid sponge Callipelta sp., collected off New Caledonia. Structural assigment was accomplished through extensive 2D NMR spectroscopy. The complete relative stereochemistry is proposed from the analysis of ROESY and NOE difference experiments. Callipeltoside A (1) represents the first member of a new class of marine-derived macrolides, containing unusual structural features including a 4-amino-4,6-dideoxy-2-O,3-C-dimethyl-alpha-talopyranosyl-3,4-urethane unit.