A methoxylated fatty acid that inhibits phospholipase A(2) (PLA(2); EC 3.1.1.4) was purified from the brown seaweed Ishige okamurae. Approximately 8.1 mg of the inhibitory compound, 7-methoxy-9-methylhexadeca-4,8-dienoic acid, was isolated from 1 kg of I. okamurae powder. Recombinant PLA(2) derived from the pathogenic bacterium Vibrio mimicus was used as the target enzyme. The methoxylated fatty acid compound competitively inhibited PLA(2) with a Ki value of 3.9 microg/mL. The concentrations required for 50% inhibition of PLA(2), oedema and erythema were 1.0 microg/mL, 3.6 mg/mL and 4.6 mg/mL, respectively. The compound strongly inhibited PLA(2) activity in vitro and had potent antiinflammatory activity in vivo.