Chattonella antiqua led to the isolation of an active fraction consisting of a mixture of four isomeric compounds. The active compounds were identified as (E)-9-hydroxyoctadec-10-enoic acid (1), (E)-10-hydroxyoctadec-8-enoic acid (2), (E)-11-hydroxyoctadec-12-enoic acid (3) and (E)-12-hydroxyoctadec-10-enoic acid (4) by NMR, IR and mass spectral data. The structures were confirmed by comparison of the NMR