Abstract. New observations of sub-cyclotron frequency instabilities in low aspect ratio plasmas in the National Spherical Torus experiment (NSTX) are reported. The frequencies of observed instabilities scale with the characteristic Alfven velocity of the plasma. A theory of localized Compressional Alfven Eigenmodes (CAE) and Global shear Alfven Eigenmodes (GAE) in low aspect ratio plasmas is presented to explain the observed high frequency instabilities. CAE's/GAE's are driven by the velocity space gradient of energetic super-Alfvenic beam ions via Doppler shifted cyclotron resonances. Properties of such instabilities are investigated.