We present an experimental observation of the dynamics of an initially chirped optical soliton at 1.55 mu m that is propagating through a single-mode optical fiber, using frequency-resolved optical gating (FROG). FROG permits observation of both the amplitude and the phase profiles of ultrashort pulses, providing complete information on the pulse evolution. The features that are detected, which include what is believed to be the first experimental observation of phase slips, are in quantitative agreement with numerical simulations that employ the nonlinear Schrodinger equation. (C) 1999 Optical Society of America OCIS codes: 060.5530, 060.7140, 320.7100, 320.7140, 320.5550.