The stability and evolution of ISX‐B‐like plasmas are numerically studied using a reduced set of resistive magnetohydrodynamic equations. For a sequence of equilibria stable to ideal modes, then= 1 mode changes from a tearing branch to a pressure‐driven branch as &bgr;pis increased. When this mode is unstable at low &bgr;, it is just the (m= 1;n= 1) tearing mode. Highernmodes also become linearly unstable with increasing &bgr;p; they are essentially pressure‐driven and have ballooning character. For low values of &bgr;, the instability is best described as a &bgr;pdistortion of the (m= 1;n= 1) tearing mode. This mode drives many other helicities through toroidal and nonlinear couplings. As &bgr;pis increased, the growth of them= 1 island slows down in time, going from exponential to linear before reconnection occurs. If &bgr;pis large enough, the island saturates without reconnection. A broad spectrum of other modes, driven by the (m= 1;n= 1) instability, is produced. These results agree with some observed features of magnetohydrodynamic activity in ISX‐B.