A study is made of the influence of nonlinearity upon the structure of a steady-state disturbance brought about by low-frequency (ω[Lt ]ωi) monochromatic magnetosound propagating in a transversally inhomogeneous low-pressure plasma (β[Lt ]me/mi), in the vicinity of the Alfvén resonance point for the case when the principal linear mechanism for forming the transverse structure is provided by magnetic field diffusion due to small plasma resistivity. It is shown that the nonlinear properties of the fundamental harmonic of a disturbance are determined by its self-action through a plasma density variation that it produces. The disturbance structure in the region of the Alfvén resonance is described.