This study examines the effect of thermal hysteresis proteins (THPs) from the winter flounder (Psuedopleuronectes americanus ) on the ice-water interface morphology during freezing of aqueous solutions. Experiments were performed using a directional solidification stage, and the development of the two-phase interface was observed through a microscope and recorded by a video system. Unusual ice crystal morphologies were observed, including faceted ice crystal growth along the  crystal plane; spicular or needlelike growth in the  direction; and growth parallel to the c -axis, , consisting of incorporated liquid inclusions bounded by hexagonal prism faces. The observed crystallographic structures can be explained as an effect of the interaction between the THPs and the primary prism faces of ice crystals. This results in an increase in the Gibbs free energy of these planes, followed by ice growth into the supercooled liquid adjacent to these faces.