## Localization of Bosonic Atoms by Fermionic Impurities in a Three-Dimensional Optical Lattice

We observe a localized phase of ultracold bosonic quantum gases in a 3-dimensional optical lattice induced by a small contribution of fermionic atoms acting as impurities in a Fermi-Bose quantum gas mixture. In particular, we study the dependence of this transition on the fermionic {sup 40}K impurity concentration by a comparison to the corresponding superfluid to Mott-insulator transition in a pure bosonic {sup 87}Rb gas and find a significant shift in the transition parameter. The observed shift is larger than expected based on a simple mean-field argument, which indicates that disorder-related effects play a significant role.