Polarization-resolved lateral-photoconductivity measurements are reported on device structures made of GaAs/Al0.3Ga0.7As quantum wells sandwiched between low-temperature grown GaAs(001) layers. The mesa device structures have long length (3 mm||y) and narrow width (10 and 20 μm||x) in the (001) plane. For light incident along , the ground state light-hole exciton transition is much stronger for light polarization E||x, compared to E||y. The heavy-hole exciton transition shows a weaker polarization anisotropy of opposite sign, being stronger for E||y. Through calculations based on the Bir–Pikus Hamiltonian, the observed in-plane optical polarization anisotropy is shown to arise from valence band mixing induced by anisotropic strain in the plane of quantum wells.