High resolution measurements of emitted wavelength versus temperature and current are reported, in four different laser structures. In the region from below to well above threshold, varying degrees of wavelength clamping through carrier density effects are seen for the first time. The anomalous wavelength shifts are explained on the basis of lateral optical field nonuniformities and this hypothesis is tested through fabrication of a specially chosen laser structure. The results are important to optical communication and especially to the construction of laser sources which are intrinsically wavelength stable. © 1984.