The fatty acid composition of the tropical genus Halimeda (Halimediaceae, Bryopsidales), represented by the six Caribbean species Halimeda opuntia (Linnaeus) Lamouroux, Halimeda goreauii (W. R. Taylor), Halimeda copiosa Goreau et Graham, Halimeda monile (Ellis et Solander) Lamouroux, Halimeda simulans Howe, and Halimeda incrassata (Ellis) Lamouroux, is reported for the first time. The six species possessed similar fatty acid compositions with some exceptions. Principal fatty acids were 14:0, 16:0, 18:2 ( n -6), and 18:3 ( n -3), followed by 20:4 ( n -6), and 20:5 ( n -3). These species contained only 16:3 and no 16:4 as the key C 16 polyunsaturated fatty acid. In the species of Halimeda studied the 18:1 ( n -7) /18:1 ( n -9) ratio was less than 1, with the exception of H. monile where it was close to 1. The very long-chain fatty acids tetracosanoic acid (24:0), pentacosanoic acid (25:0), and hexacosanoic acid (26:0), were identified in H. copiosa , H. monile , and H. incrassata . The ∆5,9 fatty acids 18:2 ( n -9) and 20:2 ( n -11) ( H. monile ) plus 26:2 ( n -17) ( H. copiosa ) were identified in algae for the first time.