We propose a stable multi-longitudinal Brillouin/semiconductor fiber laser (BSFL) as the upstream source in a bidirectional single-fiber wavelength-division multiplexing-passive optical network (WDM-PON). The downstream wavelength serves as the pump of the BSFL. Brillouin-frequency-shifted (~10.8 GHz) upstream carrier is generated to suppress the Rayleigh backscattering and back re°ection-induced crosstalk. The stable multi-longitudinal operation of the BSFL, attributed to the four-wave mixing (FWM) effect in the semiconductor optical amplifier (SOA) reduces the difficulty of generating a stable single-longitudinal fiber laser-based upstream carrier. Bidirectional symmetric transmission at 10 Gb/s over a 12.5-km single mode fiber with less than 2-dB power penalty is demonstrated.