A novel lipoxygenase was found in sardine skin. It oxidized linoleic acid more efficiently than arachidonic acid or eicosapentaenoic acid. Esterified fatty acids such as methyl linoleate and trilinolein were also oxidized. 13(S)-Hydroperoxyoctadeca-9(Z),11(E)-dienoic acid was produced as the primary oxidation product from linoleic acid by both the crude extract of skin tissue and the active fraction after gel filtration. The enzyme had an optimum pH at 7.0 and was stable over the pH range 6-9. It may participate in the initiation of lipid oxidation in fish.