When fresh fish muscle (Ophiodon elongatus) was exposed to free fatty acids (safflower oil hydrolysate), the whole myofibril lost its suspensibility more readily than its component proteins lost their solubility. SDS-acrylamide gel electrophoresis indicated that tropomyosin was the most readily insolubilized protein. In the presence of 0.04% formaldehyde, effects of the free fatty acids were increased approximately threefold.