A fast electron beam is injected into an essentially collisionless magnetoplasma. The beam current closes via a field‐aligned return current which drives an ion acoustic instability. The observed frequency spectrum is close to that predicted by renormalized turbulence theory. Three‐dimensional cross‐correlation measurements exhibit a wave vector spectrum extending in magnitude tok&lgr;D≳1 and covering in direction a half‐space centered around the electron drift. Propagation of phase coherent test waves confirm the three‐dimensional character of the instability but also indicate the presence of ion drifts. These can arise from ion trapping in observed large amplitude off‐resonant beat modes generated by two obliquely propagating unstable ion acoustic waves.