The theory of cross‐field ion streaming instabilities is applied to the parameter regime of the end cells of the upgraded Tandem Mirror Experiment with neutral beam injection. The cross‐field ion drift as well as the electron thermal anisotropyTe⊥>Te∥provides the free‐energy that drives various instabilities. Three instabilities, a nearly perpendicular propagating modified two‐stream instability, an obliquely propagating ion–ion streaming instability, and an obliquely propagating electromagnetic lower‐hybrid instability, have been identified. The first two waves are electrostatic and have the largest growth rate. For the actual operation conditions of the tandem mirror machines, it is found that the ion–ion streaming instability has the largest growth rate. When more energetic neutral beams become available, the two‐stream instability may play the dominant role in the stability of these devices.