Using a low-temperature microphotoluminescence method, we have investigated the optical properties of oval defects in a GaAs layer grown by molecular beam epitaxy. The photoluminescence (PL) spectra of oval defects exhibited new distinct peaks, which had a narrow width (0.5 meV) and a strong intensity comparable to the exciton luminescence from a defect-free region of the epilayer. The excitation-power dependence and our PL image measurements suggest that the peaks are due to the recombination of excitons bound to the defects. The PL image of the free-exciton luminescence clearly revealed the features of a pair of asymmetric oval defects, each of which had a pyramidal structure consisting of stacking fault planes. The clear features indicate effective carrier confinement within the pyramid, where the stacking fault planes functioned as a potential barrier. ©1998 American Institute of Physics.