A glycosidic macrolide, lyngbouilloside (1), was characterized from the marine cyanobacterium Lyngbya bouillonii collected from Papua New Guinea. Its structure was elucidated by spectroscopic analysis and chemical derivatization. Relative stereochemistry was deduced from homonuclear and heteronuclear coupling constants as well as NOE information. Lyngbouilloside was modestly cytotoxic to neuroblastoma cells (IC(50) value of 17 microM).