The resistive tearing mode is analyzed in the nonlinear regime; nonlinearity is important principally in the singular layer aroundk·B = 0. In the case where the resistive skin time&tgr;sis much longer than the hydromagnetic time&tgr; H, exponential growth of the field perturbation is replaced by algebraic growth liket2at an amplitude of order(&tgr; H / &tgr; S )4/5. Application of the theory to the unstable tearing modes of a tokamak with a shrinking current channel yields good agreement with the observed amplitudes of them ≥ 2oscillations. The analysis excludes the very long wavelength mode, andm = 1in the tokamak, for which the “constant‐&PSgr;” approximation is invalid.