We present a self-consistent theory and the results of numerical simulations describing the excitation and nonlinear evolution of left-hand polarized Alfven waves in the solar wind near comet P/Grigg-Skjellerup. During the Giotto spacecraft encounter with this comet, the interplanetary magnetic field was almost perpendicular to the solar wind flow, so the newly created (by photoionization) cometary ions excited left-hand polarized waves through normal Doppler resonance. In our calculations we take into account two types of nonlinearity (1) the nonlinearity of the cometary ions' motion due to their resonant interaction with excited waves (i.e., trapping and bounce oscillations), and (2) the nonlinearity of the solar wind particle motion that leads to the steepening of the wave profile. Results of the study are in reasonable agreement with observations. This analysis may also be relevant to the related problem of nonlinear wave excitation by reflected protons near planetary bow-shocks. ¿ American Geophysical Union 1995