The rate at which a single nonequilibrium electron in GaAs loses energy to an optically injected, thermalized electron-hole plasma has been measured by comparing the emission from nonequilibrium electrons recombining at neautral acceptors in the presence and absence of the plasma. The intensity of the luminescence decreases in the presence of the plasma due to carrier-carrier scattering. The decrease directly gives the scattering rate using the polar scattering rate as an internal clock. The scattering rate is linear with injected density. At the maximum density achieved (n=p=8×1016 cm-3), the rate is found to be the same as the polar scattering rate. Both single-particle scattering and collective excitations of the plasma are shown to be important. © 1989 The American Physical Society.