[en] It is shown that two-fluid plasma equations can be derived using variational principles provided the plasma velocities are irrotational. Whitham's variational method of averaged Lagrangian for the nonlinear wave theory has been slightly reformulated so as to weak nonlinear interactions among electrostatic waves. A pertubational analysis of the averaged Lagrangian yields coupled equations which describe the evolution of the interacting waves and cover all the different cases of resonant interactions inclusively. It is also shown that the nonlinear dispersion relation of a system can be conveniently derived with the averaged Lagrangian. Self-interaction of a Langmuir wave and its modulational instability are considered along this approach. (Author)