p(+)-AlGaAs is much more susceptible than n(+)-AlGaAs to the introduction of electrically active deep levels during exposure to electron cyclotron resonance Ar plasmas. In both materials the resistivity of thin (similar to 0.5 mu m) epitaxial layers increases rapidly with both plasma exposure time and the ion energy, while the ion density in the Ar discharge has a much greater influence on p(+)-AlGaAs than n-type material. These results suggest that the energetic ion bombardment introduces deep hole traps more readily than deep electron traps in AlGaAs and that pnp transistor structures are more susceptible to plasma damage than comparable npn structures.