Output spectra for narrow stripe, PbSnTe buried heterostructure laser diodes are presented and it is shown that the lasers operate in a single longitudinal mode of the fundamental transverse mode family to approximately twice threshold after which multimode operation predominates. These observations are interpreted in terms of the spectral inhomogeneity of the gain function. The small inhomogeneity arises from an intraband relaxation time of the order of 2 ps.