The operation of an ultra-narrow bandwidth optical filter based on the 5P_(3/2)-->5D_(3/2) excited-state transition in rubidium vapor is reported. The 5D_(3/2) state is excited by a circularly polarized pump beam at 780 nm from a diode laser. The filter displays a single 398-MHz bandwidth at a peak transmission of 9.0%, which is narrower than the Doppler bandwidth. The dependence of peak transmission on the pump intensity and cell temperature is also given.