An electromagnetic wave having its wave vector parallel to the direction of the steady magnetic fieldHˆis attenuated in plasma very effectively when the frequency &ohgr; is equal to the ionic resonance frequency &OHgr;gof the plasma. A small localized periodic disturbance initiated in a plasma in thermodynamic equilibrium att= − ∞ and having frequency &ohgr; = &OHgr;gproduces along the direction of the magnetic field a wave motion characterized by a complex wave vectork∼ (&OHgr;0&OHgr;g/c2u0i)⅓(√3 +i) where &OHgr;0is the Langmuir frequency andu0iis the mean thermal velocity of the ions in the plasma. The attenuation per wavelength at the resonance frequency is substantial since Imk/Rek= 1/√3. However, the attenuation per unit of length is not at maximum at ionic resonance since it increases for increasing frequencies when &ohgr; passes through the resonance. This is shown by the fact that for &ohgr; = &OHgr;gwe have Im(dk/d&ohgr;) > 0 since at resonance Im(dk/d&ohgr;) =K1/u0i+ (K2/u02ui0)⅓whereK1andK2are appropriate positive constants andu0is the velocity of the magnetohydrodynamic wave.