We report the generation of a stable single longitudinal mode in the transient spectrum of a GaAlAs laser diode using dispersive external feedback. The characteristics of the spectral evolution subsequent to a step increase in the drive current can be adequately understood from consideration of temperature induced shift of the gain spectrum and longitudinal modes as a function of time. Operation in a single longitudinal mode is observed from the first few nanoseconds out to steady state for lasers with both index guiding and gain guiding. Over 7 mW of single mode power is observed with an index‐guided laser.