The operation of a narrow-linewidth optical f ilter based on the 4P((1/2)) ? 8S((1/2)) excited-state transition in potassium vapor is reported. The 4P((1/2)) state is excited by a circularly polarized, 769.9-nm, 10-ns pulse from a dye laser. A linearly polarized, time-sequenced, and spatially overlapped probe pulse at 532.33 nm completes the transition to the 8S((1/2)) state. The peak filter transmission is ~40% with a bandwidth of less than 4 GHz. Corroborative experimental results suggest that the rotation of probe-pulse polarization by an induced circular birefringence is the dominant mechanism behind the filter operation.