Electrostatic instabilities associated with the bounce frequency are studied for a mirror‐confined plasma. A plasma with a monochromatic energy distribution perpendicular to the magnetic field is shown to introduce a new mode at &ohgr;∼n&ohgr;bwhich is unstable when&ohgr;p/&ohgr;b≳ 0(1), where &ohgr;band &ohgr;pare the bounce and plasma frequencies, respectively.