The self‐consistent rf electric field in ion cyclotron excitation of a multispecies plasma is measured and compared with linear plasma theory. For wave propagation perpendicular to the magnetic fieldBz(kz=0), the rf electric field has minima at major and minor ion cyclotron frequenciesf1,f2, respectively. Nearf&agr;, wheref&agr;is the ion cyclotron frequency of species &agr;, the field is proportional to &Dgr;f=f−f&agr;. The line width aboutf&agr;is proportional tor&agr;≡ the ratio of the ion concentrationn&agr;to the total ion densityn0. As the concentration of minor ion species decreases (r2≲ 10%), the field at frequencies nearf2increases sufficiently to accelerate the minor ions to high energy (≳ 100 eV).