We have made comparisons between the equivalent electron number density calculated from the ion instrument on the Active Magnetospheric Particle Tracer Explorer United Kingdom Satellite (AMPTE-UKS) and the electron number densities calculated with the wave instrument on AMPTE-UKS and the passive sounder on ISSEE 1. We make comparisons between the density values determined from the ion and wave instruments according to the location of the spacecraft in the foreshock region. If the spacecract is on the edge of the electron foreshock where the spectrum is narrow, the emissions are at the plasma frequency so that a direct comparison is possible (region 1). Further into the foreshock region, the spectrum is wider with a low-frequency cutoff at the plasma frequency. In this region the density from the ion instrument must be compared with the lower edge of the wave band (region 2). Still further downstream of the foreshock boundary, the spectrum intensity is distributed symmetrically above and below the plasma frequency. In this region the density from the ion instrument must be compared with the center of the wave band (region 3). Taking this into account we can make the following conclusions. For low number densities (~3 cm-3) and medium number densities (~7 cm-3) the agreement is good, within 5%. For higher number densities (~ 20 cm-3) the values from the ion instrument are within 15% of the number density values estimated from the AMPTE-UKS wave instrument. ¿American Geophysical Union 1990