Experiments with yeast plasmalemma fracture faces, produced at -196 degrees C and exposed to pure water vapor before replication, showed a "specific decoration" with ice crystals of those pits in the extraplasmatic face where the matching particles of the plasmatic face had been removed (H. Gross et al., J. Cell Biol. 79 (1978) 646). THe preferred growth of cubic ice crystals on the small depressions of the extraplasmatic face could be caused by the cavity-like character of the surface relief or by physicochemical surface properties which differ from those of the surrounding lipid fractures face. The following results suggest that not geometric or topographic but physicochemical surface properties are responsible for the trapping of water molecules in the regions of the depressions. Freeze-fractured multilamellar stearic-acid crystals and cardiolipin liposomes exhibit after fracturing hydrophobic faces and hydrophilic steps (bimolecular steps with two polar head groups in half of the step height) which act as preferential nucleation sites and lead to the formation of discrete ice crystals mainly along the steps. Multilamellar crystals of the paraffin n-octatriacontane (C38H78) exhibit after freeze-fracturing hydrophobic faces and geometrically identical, but hydrophobic steps. In contrast to stearic acid these steps do not act as preferential nucleation sites.