An experiment simulating certain characteristics of the equatorial electrojet plasma (viz. &ngr;e≪&OHgr;e&ngr;i>&OHgr;i) is described. High frequency (&ohgr;>&ngr;i>&OHgr;i) instabilities are observed when electrons stream in the E¿B direction past ions with a velocity exceeding the ion acoustic speed. The observed phase velocities and frequencies agree with the results of the Farley-Buneman instability theory, modified to take account of finite k∥. The measured instability spectrum for potential fluctuations goes as k-n⊥ with n=2-3.5.