From the dinoflagellate Coolia monotis, a new ceramide (1) bearing a 2-hydroxy-15-methyl-3-octadecenoyl moiety was isolated as a cellular constituent. The structure, including the partial absolute configuration, was elucidated as 1 on the basis of the 2D NMR of 1, and chiral HPLC and CD examinations with benzoyl derivatives of its degradation products. To our knowledge, this is the first example, from natural sources, of a C18 fatty acid with a methyl group substituted at a C15 carbon.