Glycolipids of Myrmekioderma sponges contain Myrmekiosides, a new family of glycolipids with a unique structure of mono-O-alkyl-diglycosylglycerols. This report deals with the identification and biological activity of the new Myrmekioside E from Myrmekioderma dendyi. Its structure has been elucidated from spectroscopic data and chemical degradation studies. It contained a glycerol backbone linked to xylose and N-acetylglucosamine, and an alkyl long-chain with a terminal alcohol group. A related glycolipid, Trikentroside, known in the sponge Trikentrion laeve, was subjected to a comparative biological evaluation. Both glycolipids inhibit proliferation of two human non-small cell lung cancer cell lines (NSCLC-N6 and A549).