An anisotropic Maxwellian velocity distribution with loss cones is used to investigate cyclotron wave instabilities occurring in a magnetic mirror configuration. The density of electrons in the loss cones is assumed to be negligible; this implies that the particle velocity distribution function is such thatf0(v⊥,vz) = 0 for |vz| >v⊥(R− 1)½, wherevzis the component of particle velocity along the magnetic field,v⊥ is the component of particle velocity transverse to the magnetic field, andRis the mirror ratio. Linearized Vlasov equations are solved to obtain the dispersion relation for transverse electromagnetic waves propagating along a uniform magnetic field. Numerical solutions of the growth and damping rates of the wave for a mirror ratio of 2.0 show that there is a long‐wavelength instability for a Maxwellian electron distribution with loss cones. The applicability of the analysis to a finite‐length mirror configuration is discussed.