The antifreeze polypeptide (AFP) from the winter flounder displays partial alpha-helix formation at lower temperatures. To investigate the relationship between antifreeze activity and alpha-helical structure, we designed and then chemically synthesized an AFP analog with enhanced alpha-helicity, and compared its conformation and antifreeze properties with those of the native AFP. The synthetic analog was more helical than the native AFP; however, the antifreeze activity of both peptides were identical. The antifreeze activity of the peptides displayed a strong pH dependence, which paralleled pH-induced changes in helix content. At pH 8.5, the antifreeze activity of both peptides displayed identical concentration dependences. In addition to antifreeze activity measurements, the effects of the peptides on the rate of ice crystal growth were also measured. While both peptides affected the a- and c-axis growth rates of ice crystals, the highly helical analog was able to exert its effect on ice crystal growth rates at 7-8-fold lower concentrations than the native AFP. These data indicate that there is a direct but complex relationship between alpha-helicity and antifreeze activity.