The noncolligative peptide and glycopeptide antifreezes found in some cold-water fish act by binding to the ice surface and preventing crystal growth, not by altering the equilibrium freezing point of the water. A simple crystal growth and etching technique allows determination of the crystallographic planes where the binding occurs. In the case of elongated molecules, such as the alpha-helical peptides in this report, it also allows a deduction of the molecular alignment on the ice surface. The structurally similar antifreeze peptides from winter flounder (Pseudopleuronectes americanus) and Alaskan plaice (Pleuronectes quadritaberulatus) adsorb onto the (2021) pyramidal planes of ice, whereas the sculpin (Myoxocephalus scorpius) peptide adsorbs on (2110), the secondary prism planes. All three are probably aligned along (0112). These antifreeze peptides have 11-amino acid sequence repeats ending with a polar residue, and each repeat constitutes a distance of 16.5 A along the helix, which nearly matches the 16.7 A repeat spacing along (0112) in ice. This structural match is undoubtedly important, but the mechanism of binding is not yet clear. The suggested mechanism of growth inhibition operates through the influence of local surface curvature upon melting point and results in complete inhibition of the crystal growth even though individual antifreeze molecules bind at only one interface orientation.