The probability of positronium (Ps) formation in semicrystalline poly(ethylene terephthalate) (PET), has been investigated by means of positron annihilation lifetime spectroscopy (p.a.l.s.). The p.a.l.s. data are correlated with the structural characteristics of the samples, such as crystalline content, interlamellar spacing and lamella thickness. The results show an approximately linear decrease of the intensity (I-3) of the long-lived orthopositronium (o-Ps) component with an increase in the crystalline content up to 53%, while the lifetime remains practically constant. It seems that the other structural parameters do not affect the Ps formation. The decrease of I-3 cannot be completely accounted for if one assumes that o-Ps localizes in the amorphous phase only. It is suggested that the o-Ps also forms in the crystalline regions, due to the inhomogeneous electron density distribution along the c-axis of the PET lattice.