The phase velocities of 1-m electrostatic plasma waves excited by the two-stream instability in the ionospheric E region were measured with a coherent radar. Simultaneously, plasma temperatures were measured with an incoherent radar inside the sampling volume of the coherent radar. The phase velocities, when measured in units of the ion acoustic velocity, were shown to decrease in magnitude from 1.0 toward 0.7 as the electron temperature of the plasma increases from 500¿ to 3000¿K. Recent theoretical predictions are in accord with this experimental result. ¿ American Geophysical Union 1995