The stretching-induced alpha-beta polymorphic transition in the crystalline regions of poly(butylene terephthalate) (PBT) is studied by small-angle X-ray scattering (SAXS), instead of by the wide-angle X-ray scattering (WAXS) usually used for this purpose. An expression for the increase of the long spacing upon the transition is derived from a simple model of oriented PET as a set of microfibrils, each consisting of alternating crystalline and amorphous regions. This increase is calculated on the basis of the c unit cell parameter of both alpha and beta phases and the degree of crystallinity as obtained from differential scanning calorimetry. There is a fairly good agreement between this value and the experimentally measured one. The affine deformation model holds for both the alpha and beta phases in the whole interval of the strain, from zero up to the strain at break. Without external stress the microfibrils are in the fully relaxed state, as concluded from the values of the corresponding long spacing (from SAXS) comprising exclusively the alpha modification (from WAXS). Both the polymorphic transition and the change in the long spacing with strain are completely reversible in the deformation range investigated.