On an instrumented rocket payload flown through the polar ionosphere, large-amplitude electrostatic ELF waves have been measured in the unstable E region. Using three-axis instantaneous measurements of the electric vector of the wave field, it was possible to confirm the electrostatic nature of the waves and to make a direct determination of the full k vector as a function of altitude during upleg and downleg passages through the E region. Simultaneous measurements were made of electron density and temperature, de electric field, and ac and dc magnetic field. This allows comparisons to be made with theories of the crossed field two-stream instability, the conditions for which were well satisfied. The direction of propagation and the frequency range are in agreement with theory, but the phase velocity is lower than predicted, being only about one third of the electron drift velocity.