The differential external quantum efficiency η
extof Fabry-Perot-type GaAs injection lasers has been measured as a function of the mean reflectivity

. By evaporation of antireflective films,

has been varied between 0.25 and 0.06. The observed increase of η
extwith decreasing

is interpreted in terms of a simple model. Values for the optical losses α, the gain factor β, and the internal quantum efficiency at 77°K are obtained for individual diodes.