Eicosanoids are oxygenated derivatives of C20 polyunsaturated fatty acids known to play key roles in many physiological events in both invertebrates and vertebrates. The eicosanoid generating capacity of cypris larvae of the barnacle, Balanus amphitrite, was examined using enzyme immunoassay and high-performance liquid chromatography. These larvae generated the lipoxygenase products, 12-hydroxyeicosapentaenoic acid (HEPE), 8-HEPE and 8,15-diHEPE, together with the cyclooxygenase products, prostaglandin (PG) E, PGF and thromboxane (TX) B. Indomethacin, a selective cyclooxygenase inhibitor, caused a dose-dependent inhibition of PGE generation by B. amphitrite larvae, while esculetin and nordihydroguaiaretic acid (lipoxygenase inhibitors) also strongly inhibited the generation of 8-HEPE, 12-HEPE and 8,15-diHEPE. PGE2, PGE3 and 16,16-dimethyl PGE2 caused a dose-dependent inhibition of settlement of B. amphitrite larvae while indomethacin (25–100 μM) stimulated this process. Lipoxygenase products (8-HEPE, 12-HEPE and 8,15-diHEPE) as well as esculetin and nordihydroguaiaretic acid (10–100 μM) had no effect on the attachment of larvae.