Two parallel, relativistic electron streams with different velocities support unstable, exponentially growing space‐charge waves, and efficient electron bunching may be achieved in the submillimeter wavelength range. Injection of the prebunched streams into a static, periodic (wiggler) magnetic field enhances the intensity of the parametrically excited, backscattered electromagnetic wave. Calculations are presented of the amplitude enhancement and of the linear growth rate of the wave in the Raman regime, for the case of two cold, perfectly intermingled electron streams.