Structure studies on the segmented polyurethane-urea based on poly(tetramethylene glycol), 4,4'-diphenylmethane diisocyanate, and 4,4'-diaminodiphenylmethane have been made using wide- and small-angle x-ray diffraction. It was found that the hard segments have a well-defined crystal structure indexed by a monoclinic unit cell with dimensions a = 4.72 Å, b = 11.33 Å, c (chain axis) = 11.64 Å, and γ = 116.5°. Further, the hard segments have an anisotropic supermolecular structure, that is, a spherulitic crystalline texture. The microdomain structure depends on the molecular weight of the soft segments, with phase separation becoming distinct with an increase in the molecular weight of the soft segment. Based on the long period observed in small-angle x-ray patterns of heat-set specimens, the thickness of lamellae was estimated to be about 93 Å. This corresponds to an average hard segment sequence length of 8. On deformation, the hard segment orients preferably transverse to the stretch direction up to high elongations, and it shows a hysteresis of orientation. This phenomenon can be explained in terms of the anisotropic supermolecular structure. Further, deviation from ideal rubber elasticity observed in the stress-strain relationships of the segmented polyure-thane-urea elastomers might also be caused by the anisotropic structure.