  The instabilities of relativistic ion beams in a relativistically hot
electron background are derived for general propagation angles. It is shown
that the Weibel instability in the direction perpendicular to the streaming
direction is the fastest growing mode, and probably the first to appear,
consistent with the aligned filaments that are seen in PIC simulations.
Oblique, quasiperpendicular modes grow almost as fast, as the growth rate
varies only moderately with angle, and they may distort or corrugate the
filaments after the perpendicular mode saturates.
