Low-frequency electromagnetic fluctuations dominate the wave spectrum observed during strong plasma flows in the region upstream of a cometary nucleus and in the plasma sheet boundary layer of the Earth's magnetotail. A theoretical framework is established in the long-wavelength limit for nonresonant instabilities in a multibeam plasma, for any number of beams. Both the growth rates of the instabilities and the ultimate levels of magnetic field turbulence are given. A two-beam plasma models the contamination of the upstream solar wind by cometary material, whereas a three-beam model is invoked for the plasma sheet boundary layer, with two counterstreaming proton beams in the presence of an oxygen beam. If the two proton beams by themselves already excite a low-frequency instability, the injection of oxygen at rest in the center-of-mass frame does not change the instability characteristics. Where the two proton beams alone are stable, a sufficient amount of oxygen with a sufficient velocity offset in the center-of-mass frame can render the plasma sheet boundary layer unstable. In both cases, the theoretical levels of low-frequency turbulence are in good agreement with observations and/or numerical simulations. ¿American Geophysical Union 1991