Activity of antifreeze proteins (AFPs) and antifreeze glycoproteins (AFGPs) is often determined by thermal hysteresis, which is the difference between the melting temperature and the nonequilibrium freezing temperature of ice in AF(G)P solutions. In this study, we confirmed that thermal hysteresis of AFP type I is significantly enhanced by a cooperative function of ammonium polyacrylate (NH(4)PA). Thermal hysteresis of mixtures of AFP type I and NH(4)PA was much larger than the sum of each thermal hysteresis of AFP type I and NH(4)PA alone. In mixed solutions of AFP type I and NH(4)PA in the thermal hysteresis region, hexagonal pyramidal-shaped pits densely formed on ice surfaces close to the basal planes. The experimental results suggest that the cooperative function of NH(4)PA with AFP type I was caused either by the increase in adsorption sites, of AFP type I on ice or by the adsorption of AFP type I aggregates on ice.