The fundamental azimuthal modes of a constricted annular resonator are investigated. It is found that a given mode of an unconstricted resonator splits into two separate modes in the constricted resonator. One mode is of a higher frequency and has a pressure antinode centered in the constricted region. The other mode is of a lower frequency and has a pressure node centered in the constricted region. The resonance frequency of the higher-frequency modes increases linearly with a decrease in the constricted to unconstricted area ratio, whereas the lower frequency drops nonlinearly. Measurements and theory match to within 0.5% when end corrections and thermo-viscous losses are included in the system model. It was found that end correction impedances derived by mode-matching techniques were the only ones accurate enough to match the measurements and computation to within the error bounds.