[en] The electromagnetic oscillating two-stream instability of electron cyclotron waves in a plasma has been examined. The low-frequency perturbation is a purely growing electromagnetic mode having k vector parallel to the ambient B field and the sidebands are elecrostatic plasma waves. The instability is excited when either ω/sub c/, ω/sub p/> or =ω₀ > or =(ω²/sub p/+ω²/sub c//4)¹/sup //² -ω/sub c//2 or ω/sub u/>ω₀>ω/sub c/,ω/sub p/. Here ω₀ is the frequency of the pump and ω/sub c/,ω/sub p/, and ω/sub u/ are the electron cyclotron, plasma, and upper-hybrid frequencies, respectively. The threshold power, due to the finite extent of the pump, is a few hundred kilowatts for the parameters of large tori. The growth rate above the threshold γapprox. =ω₀ k₀v²/sub 0x//8k²v²/sub e/, where v/sub 0x/ and v/sub e/ are the oscillatory and thermal speeds of electrons