Antifreeze proteins (AFPs) bind to the surface of ice crystals and lower the non-equilibrium freezing temperature of the icy solution below its melting point. We have recently reported the discovery of three novel hyperactive AFPs from a bacterium, a primitive insect and a fish, which, like two hyperactive AFPs previously recognized in beetles and moths, are considerably better at depressing the freezing point than most fish AFPs. When cooled below the non-equilibrium freezing temperature, ice crystals formed in the presence of any of five distinct, moderately active fish AFPs grow suddenly along the c-axis. Ice crystals formed in the presence of any of the five evolutionarily and structurally distinct hyperactive AFPs remain stable to lower temperatures, and then grow explosively in a direction normal to the c-axis when cooled below the freezing temperature. We argue that this one consistent distinction in the behaviour of these two classes of AFPs is the key to hyperactivity. Whereas both AFP classes bind irreversibly to ice, the hyperactive AFPs are better at preventing ice growth out of the basal planes.