Simultaneous coherent and incoherent scatter measurements from a common ionospheric volume are used to investigate the relationship between the phase velocity Vph of meter wavelength irregularities generated by the two-stream instability and the ambient ion acoustic speed Cs in the auroral electrojet plasma, with the purpose of testing the widely accepted conviction that Vph is always limited at Cs. This study is representative of typical VHF electrojet backscatter at magnetic aspect angles nearing perpendicularity and electric fields varying in the 20- to 50-mV/m range. Since it is not clear is the charged particles behave isothermally or adiabatically, we used several options for Cs suggested in the literature, including (1) isothermal behavior for both ions and electrons, (2) isothermal ions and adiabatic electrons with 3 degrees of freedom, and (3) adiabatic electrons with 3 degrees of freedom and adiabatic ions with 1 degree of freedom. The comparison shows that option 3, in which &ggr;e=5/3 and &ggr;i=3, is rather unlikely. On the other hand, we find that option 2 (&ggr;e=5/3, &ggr;i=1) is more appropriate for use in the ion acoustic speed for meter wavelength waves. The analysis shows that on the average, phase velocities are limited to values which are ~15% higher than Cs in line with the nonlinear kinetic model of the modified two-stream instability developed recently by Robinson and Honary (1990). Finally, in spite of the great deal of uncertainty involved in the Vph estimates, there is convincing evidence in the data showing that at times another mechanism, other than anomalous electron gas heating, is involved which allows for Vph to receive values well above Cs. ¿American Geophysical Union 1990