Magnetohydrodynamics (MHD) turbulence at high magnetic Reynolds numbers is considered. The mean field magnetohydrodynamic equations for the rather general case of inhomogeneous turbulence are derived using the well-known Orszag scheme. Special attention is paid to nonlinear effects which are brought about by the back reaction of the magnetic field on the motion. An equation governing the quantity β, corresponding to the magnetic gyrotropy of Pouquetet al. (1976) is derived. The diamagnetic effect, that in the nonlinear regime is accompanied by a paramagnetic effect, is considered. The nonlinear transport of large-scale field due to density inhomogeneity is discussed. A physical interpretation is presented which clarifies the mechanism of field transport in an inhomogeneously turbulent medium. Some astrophysical applications are discussed.