An electromagnetic wave traveling in a multistream medium in the direction of an applied magnetic field is described by a dispersion relation as follows: $c^2{k}^2={\omega }^2-\Sigma \stackrel{}{i}\frac{{{\omega }_i}^2\omega }{(\omega -k{V}_i\mp {\Omega }_i)},$ where $\omega$ is the frequency, $k$ is the wave vector, ${\omega }_i$ is the Langmuir frequency, ${\Omega }_i$ is the gyromagnetic frequency associated with the beam $i$, and $V_i$ is the velocity of the beam $i$ ($i=1, 2, 3\cdots n$). For the case of gyromagnetic resonance ($\omega ={\Omega }_i$) we have an occurrence of growing and evanescent waves if $M=27\mathrm{}{\left[\Sigma i^{}\frac{{{\omega }_i}^2}{V_i}\right]}^2-\frac{4{{\Omega }_g}^4}{c^4}>0\mathrm{}$, where ${\Omega }_g={\Omega }_i$ for all $i\text{'}\mathrm{s}$, and the occurrence of unattenuated waves if $M\le 0$.

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