Studied were the changes taking place in the fats of frozen mackerel according to its quality. The organoleptic evaluation performed by a standard method served as a basis of comparison. It was found that the fats of standard frozen ocean mackerel show a thiobarbiturate number after Kopecky (E/n X 10) ranging up to 10, and a peroxide number of up to 0.20% J2, resp. the index after Lea (ml 0.01n Na2S2O3 per gram of fat) being up to 1.60. The fats are extracted in a way that is based on Bligh and Dyer's method.