Malyngic acid, a major fatty acid in several varieties of the marine blue-green alga Lyngbya majuscula, has been determined to be 9(S),12(R),13(S)-trihydroxyoctadeca-10(E),15(Z)-dienoic acid from chemical and spectral data and by its conversion to 9(S),12(R),13(S)-trihydroxystearic acid and degradation to 2-deoxy-D-ribitol.