The concentric interferometer with a limiting aperture in its mid-plane is analyzed for its mode-selective properties. Two of the lowest-order transverse modes and their losses for the infinite-strip geometry are computed by solving the associated integral equations by the method of successive approximations. The apertured concentric interferometer is found to be more mode-selective than the apertureless concentric interferometer or the Fabry-Perot interferometer with parallel plane mirrors. Computed results indicate that the optimum aperture size for maximum mode selectivity is approximately the size of the major lobe of the diffraction pattern of the dominant mode at the aperture plane. However, the maximum selectivity attainable does not exceed that of the “comparable” confocal system. The latter system is not very practical because it requires either very long resonator lengths or very small mirrors.