During the cold storage of cod fillets there is a gradual increase in the concentration of free fatty acids which are derived from the enzymatic hydrolysis of cod muscle phospholipid. One of the enzymes responsible, a lysophospholipase, was investigated in this study and its properties were found to be similar to those of mammalian lysophospholipases. Lysolecithin was hydrolysed to free fatty acids and glycerylphosphorylcholine; oleic acid strongly inhibited this reaction. The enzyme had maximal activity at pH 7.6 and it was deactivated by heating at 60 °C. Muscle extracts kept their activity undiminished for several weeks at −9 °C. The lysolecithinase activity of cod flesh was many times greater than its lecithinase activity. There were great, and yet unexplained, variations of activity among various cod.