Fish that live in the polar oceans survive at low temperatures by virtue of 'antifreeze' plasma proteins in the blood that bind to ice crystals and prevent these from growing. However, the antifreeze proteins isolated so far from the winter flounder (Pleuronectes americanus), a common fish in the Northern Hemisphere, are not sufficiently active to protect it from freezing in icy sea water. Here we describe a previously undiscovered antifreeze protein from this flounder that is extremely active (as effective as those found in insects) and which explains the resistance of this fish to freezing in polar and subpolar waters.