[en] The nonlinear stage of modulation instability of weakly ionized plasma is studied for long-wave perturbations kr₁ << 1 (k is the wave number, r₁ is the electron Larmor radius). The investigation is carried out in the one-dimensional model in which modulation of plasma density develops only in the direction normal to the magnetic field. The theoretical data well agree with the calculational ones. An analysis of numerical results shows that the development of thermal modulation instability in homogeneous magnetoactive plasma differs noticeably from the development of regular striction instability