[en] A model equation describing drift Alfven wave with E x B nonlinearity is derived. For a special ordering a nonlinear Schroedinger equation is derived, which governs modulational instability of the drift Alfven wave. Translational invariance is assumed along the magnetic field. The relation between the characteristic scale lengths parallel and perpendicular to the drift flow for the onset of cell formation is found to depend on if the wave propagates parallel or antiparallel to the drift flow and on the sign of the wave energy. The influence of perpendicular ion viscocity is also discussed. (author)