[en] The propagation of high-power femtosecond laser pulses is considered in the self-focusing and filamentation formation regime. Similarities and differences in the character of the laser beam filamentation are analysed qualitatively and quantitatively in the atmospheric air and water. Based on the numerical results, the most important parameters (radius, intensity, free electron density) in light and plasma filaments produced by ultrashort laser pulses are shown to depend strongly on the optical parameters of the propagation medium, and, first of all, on the Kerr nonlinearity coefficients and the photoionisation rate of the medium molecules. (nonlinear-optics phenomena)