The temperature dependence of the humidity-sensitive spacing, d , related to the lateral packing of collagen molecules was measured for fully hydrated collagen. In the vicinity of O"C, a sudden change in d was observed, which was reversible with temperature. In the diffraction profile, below O"C, a set of diffraction peaks identified with the hexagonal crystalline form of ice was observed. With the reduction in water content, the intensity of the set of diffraction peaks decreased and was found to be zero a t a water content of 0.38 g/g collagen. These results were considered to be caused by the frozen water in collagen fibril below 0Β°C. According to the water content dependence of d , it was considered that u p to a certain water content water absorbed would be stowed in the intermolecular space of collagen and above that water content water molecules would aggregate to make pools, i.e., extrafibrillar spaces. The unfreezable bound water was considered to be located in the intermolecular space of collagen. Size of the extrafibrillar space, determined from the intensity analysis of a smallangle x-ray scattering pattern, corroborates the speculation that the water showed in the extrafibrillar space is freezable and free. The formation of the hexagonal crystalline form of ice in the extrafibrillar space was considered to cause the sudden change in d at 0Β°C.