The polarization anisotropy of the photoluminescence from quasiplanar quantum wires fabricated on the sidewall of [01¯1]-oriented mesas on the (311)A GaAs surface is investigated using a modulation technique that combines high spatial (∼1 μm) and polarization resolution (≪0.1%). Due to their low symmetry, (311)A-oriented quantum wells are intrinsically anisotropic with the fundamental transition preferentially oriented along the [2¯33] direction. The anisotropy contribution from the lateral confinement in the [01¯1] wires opposes that from the vertical confinement. With increasing lateral confinement, the wire anisotropy initially reduces and then changes its sign. The lateral extent of the electronic wave functions involved in the anisotropic transitions is obtained from a comparison of the results with a theoretical model. © 1999 American Institute of Physics.