The novel alpha-hydroxy fatty acids 2-hydroxy-13-docosenoic acid [1a], 2-hydroxy-14-tricosenoic acid [2a], and 2-hydroxy-15-tetracosenoic acid [3a] were identified in the Caribbean urchin, Tripneustes esculentus. The double-bond positions of the novel alpha-hydroxy fatty acids were determined by derivatization with dimethyl disulfide and shown to correlate with the corresponding non-hydroxylated mono-unsaturated fatty acids, 13-docosenoic acid, 14-tricosenoic acid, and 15-tetracosenoic acid also present in T. esculentus. The total fatty acid composition of the urchins is also reported where cis-5-olefinic fatty acids such as 5,9-octadecadienoic acid and 5,11-eicosadienoic acid were found to predominate in the mixture. Cholesterol was the predominant sterol in T. esculentus.