The transmission properties of different types of all-silica fibers are investigated, using a multimode high-power cw Nd:YAG rod laser with 2000 W output power. The dependence of the output upon the input beam parameters, waist diameter and divergence, and the resulting power transmission are given. The upper limits for the maximum beam parameters and maximum laser powers, which can be coupled without loss into fibers with certain numerical apertures, as well as the dependence of the output beam profiles upon the intensity distribution at the fiber input are briefly discussed. An experimental set-up is presented, whereby the beam parameters are chosen and the measuring procedures are performed as suggested by the ISO. The waist diameter at the fiber end face, dependent on the laser power, is observed, so controlling the influence of the spherical aberrations of the fiber coupling lens systems. The end faces of polished and cleaved fibers are compared.