We report single longitudinal mode operation of conventional double-heterostructure GaAs/GaAlAs diode lasers achieved by heavy p-type doping of the active region. At 77 °K a broad-area device pumped with a rectangular current pulse exhibited single longitudinal mode operation up to 2.8 times threshold, which was the limit of the pulser. With a triangular pumping pulse at 300 °K, the same laser ran single longitudinal mode up to 1.4 times threshold. We believe these results can be explained by the absence of hole injection, which eliminates spatial hole burning caused by the slowly diffusing injected holes.