A diffusion equation is derived which describes the evolution of a magnetic field in a plasma of arbitrary &bgr; and resistivity. The equation is valid for a one‐dimensional slab geometry, assumes the plasma remains in quasi‐equilibrium throughout its evolution (i.e., pressure balance), and does not include thermal transport. Scaling laws governing the rate of change of the magnetic energy, particle drift energy, and magnetic flux are calculated. It is found that the magnetic free energy can be substantially larger than the particle drift energy and can be an important energy reservoir in driving plasma instabilities (e.g., the lower‐hybrid‐drift instability). In addition, the effect of a spatially varying resistivity on the evolution of a reversed‐field plasma is studied. The resistivity model used is based upon the anomalous transport properties associated with the nonlocal mode structure of the lower‐hybrid‐drift instability. The relevance of this research to laboratory plasmas (e.g., theta pinches, reversed‐field theta pinches) and space plasmas (e.g., the earth’s magnetotail) is discussed.