[en] The ideal magnetohydrodynamic stability of high-beta model equilibria in slab geometry with B/sub z/ = 0 and containing elongated regions of closed field lines is investigated as an initial-value problem and in terms of the energy principle. For moderate or larger elongations, flute (interchange) modes are found to be stable throughout the closed region. Co-interchange modes with the character of an m = 1 kink mode, however, are found to be unstable. The growth rate of these modes initially increases rapidly with k/sub z/ and then approaches a finite limit as k/sub z/→infinity. As the elongation of the closed region is increased, the growth rate decreases rapidly. Possible implications of these results for field-reversed theta pinches are discussed