Lasing spectra of a transverse‐mode‐stabilized AlGaAs laser of a channeled‐substrate planar structure have been investigated. These lasers, which oscillate in the fundamental transverse mode, reproducibly operate in a single longitudinal mode. In addition, their linewidth is as narrow as or smaller than 30 MHz when the injection current is ∼1.2 times above threshold. The intensity distribution and excitation dependence of a nonlasing longitudinal mode have been found to be more complicated than expected from a simple theory: sizable dips have been observed in the envelope function, and nonlasing modes have been found to decrease as the injection current is increased above threshold. Furthermore, hysteresis has invariably been observed in the lasing‐wavelength–vs–device‐temperature (or dc current) characteristics. These behaviors are believed to reflect a slightly inhomogeneous nature of the gain spectrum because of a finite thermalization time of injected carriers, as discussed recently by Yamada and Suematsu.