The brown alga Dictyota bartayresiana collected in the Gulf of Mannar of the Indian Ocean yielded diterpenes consisting of one known dolastane, five known dolabellanes and five new compounds that were characterized as $(15, 3E, 5R, 7E, 11S, 12S)$ -5-acetoxy-12-hydroxydolabella-3,7-dien-9-one, $(15, 3E, 5R, 7Z, 11S*)$ 12S*)-5-acetoxy-12-hydroxydolabella-3,7-dien-9-one, (1R*,3Z,7E,11S*)-9-acetoxydolabella-3,7,12-trien-16-al, (5R*, 8S*,9S*,12S*,14S*)-9-hydroxydolasta-1,3-dien-6-one (1R*,4S*,5R*,8S*,9S,12S*,14R*)-trihydroxydolasta-2-en-6-one, by interpretation of high resolution spectral data and chemical interconversions.