Electric field induced changes in the optical properties of poly(3-octylthiophene) in the spectral region of the lowest pi-pi* electronic transition are reported. The electroabsorption [EA] resembles the first energy derivative of the unperturbed spectrum, scales quadratically with the applied electric field, and its lineshape is field invariant, all consistent with a DC Kerr effect whose origin is the Stark shift of a singlet exciton. In addition, a transition that does not appear in the unperturbed absorption spectrum is detected and we assign this to the normally dipole forbidden one-photon excitation of a (1)A(g) even parity exciton that becomes allowed by the symmetry breaking effect of the applied field. The optical constants of P3OT have also been determined and these are used to calculate the real and imaginary parts of the DC Kerr susceptibility, chi((3))(-omega;0,0,omega) from the EA spectrum.