We have measured the optical gain and carrier lifetime (τ) at threshold in AlGaAs single quantum well lasers in the temperature range10-70degC. 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 gainGis found to vary linearly with the currentI. The observed slow decrease ofdG/dIand 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.