Using Brillouin spectroscopy (BS), the tensor of the elastic constants of oriented poly(ethylene terephthalate) was determined for a variety of morphologies obtained by different uniaxial drawing procedures. The extreme values of the moduli along the drawing direction at frequencies of a few gigahertz were C-33 = 40 GPa and C-44 = 1.8 GPa. As a result of the invariants of the single-phase aggregate model, the oriented state is dominated by the Reuss average even at extreme draw ratios and subsequent to a deformation-induced crystallization. This is documented in both the BS orientation parameter and the BS mode numbers in comparison with birefringence. Additional spectral lines observed at draw ratios larger than 6 are discussed in relation to the formation of nanostructured phases.