Ice growth in a cold environment is fatal for polar organisms, not only because of the physical destruction of inner cell organelles but also because of the resulting chemical damage owing to processes such as osmotic shock. The properties of ice-binding proteins (IBPs), which include antifreeze proteins (AFPs), have been characterized and IBPs exhibit the ability to inhibit ice growth by binding to specific ice planes and lowering the freezing point. An ice-binding protein (FfIBP) from the Gram-negative bacterium Flavobacterium frigoris PS1, which was isolated from the Antarctic, has recently been overexpressed. Interestingly, the thermal hysteresis activity of FfIBP was approximately 2.5 K at 50 mM, which is ten times higher than that of the moderately active IBP from Arctic yeast (LeIBP). Although FfIBP closely resembles LeIBP in its amino-acid sequence, the antifreeze activity of FfIBP appears to be much greater than that of LeIBP. In an effort to understand the reason for this difference, an attempt was made to solve the crystal structure of FfIBP. Here, the crystallization and X-ray diffraction data of FfIBP are reported. FfIBP was crystallized using the hanging-drop vapour-diffusion method with 0.1M sodium acetate pH 4.4 and 3M sodium chloride as precipitant. A complete diffraction data set was collected to a resolution of 2.9 A˚ . The crystal belonged to space group P4122, with unit-cell parameters a = b = 69.4, c = 178.2 A˚ . The asymmetric unit contained one monomer. To investigate the structure and antifreeze mechanism of FfIBP derived from Flavobacterium frigoris PS1, we have carried out structural determination by using X-ray crystallographic experiments