The marginal stability of MHD modes is discussed in application for high beta multiple mirror experiments planned at Budker Institute of Nuclear Physics. Flute modes are dangerous in axisymmetric systems with beta < 1. In the case of "wall confined" plasmas, (beta much greater than 1), pressure slightly varies along the radius providing less radial gradient and more stability against MHD modes. Effect of ion-ion viscosity becomes important in corrugated magnetic field. It results in the reduction of the growth rate by a factor beta(1/2). Il, the process of start up and plasma heating beta < 1. If flute modes are stabilized during this period by the line-tying mechanism ballooning modes are still unstable when beta > beta(cr). A. very low ballooning margin is predicted in multiple mirror with the large number of cells: beta(cr) less than or equal to pi(2) /N-2. For the number of cells N similar or equal to 10: beta(cr) similar or equal to 5%. Results of the calculations are discussed in the context of old and new multiple mirror experiments.