A new set of equations is obtained which describes evolution of finite amplitude, dispersive, elliptically polarized quasi-parallel Alfv¿n waves, when the plasma and the wave amplitude satisfy the condition, ‖1-C2s/C2A‖<ΔB/B0, where CA and CS are the Alfv¿n and the sound speed, respectively, and ΔB/B0 is the Alfv¿n wave amplitude normalized to the background magnetic field. When this condition is satisfied, the sound wave as well as the right- and left-hand polarized Alfv¿n waves are nearly all degenerated. The solar wind plasma is a typical medium for the obtained se of equations to apply, since the plasma &bgr; is high, and the solar wind Alfv¿n waves have large amplitude. Modulational instability of a circularly polarized parallel Alfv¿n wave is discussed. ¿ American Geophysical Union 1993