SUMMARY—The lipid composition of Baltic herring (Clupea harengus var. membranus) muscle and the hydrolysis during freeze storage was studied with thin‐layer chromatography. The concentrations of the various lipid classes were initially 2–5 times as high in dark muscle as in white muscle. Storage for up to 12 weeks at –15°C resulted in an increase of the free fatty acid content from 50 to 1000 mg/100 g in the dark muscle, and from 17 to 280 mg/100 g in white muscle. The increase was due to hydrolysis of lecithin, cephalin and to a varying extent also of triglycerides. According to indirect estimations, roughly 45 and 75% of the free fatty acid formed in dark and white muscle respectively was a result of phospholipid hydrolysis; the remainder of triglyceride hydrolysis. The hydrolysis of lecithin was faster than that of cephalin in both types of muscles. The enzymatic attack showed no preference for anyone of the different fatty acids in the phosp