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We have measured the optical gain and carrier lifetime (τ) at threshold in AlGaAs single quantum well lasers in the temperature range C. The small shift of the spectral position of the gain peak with increasing injection is evidence for the two-dimensional-like density of states in a quantum well. The net gain is found to vary linearly with the current . The observed slow decrease of and τ with increasing temperature suggests the absence of a large temperature-dependent nonradiative carrier loss or optical absorbtion loss in these lasers. This is also supported by the observed low temperature dependence of the external differential quantum efficiency.