Abstract: The growth of interference fringes caused by nonlinear propagation instabilities has been experimentally investigated using a femtosecond pulsed laser. This paper reports the measurements of growth rate for fringes in a glass cuvette filled with CS2 as a function of fringe spacing and intensity. The ripples grow rapidly when the modulated beam propagates in the nonlinear media, and there exists a modulation frequency where the intensity modulation grows fastest and filamentation happens. Numerical results show good agreement with the experimental results．