The local effects of isolated magnetic adatoms on the electronic properties of the surface of a superconductor were studied with a low-temperature scanning tunneling microscope. Tunneling spectra obtained near magnetic adsorbates reveal the presence of excitations within the superconductor’s energy gap that can be detected over a few atomic diameters around the impurity at the surface, These excitations are locally asymmetric with respect to tunneling of electrons and holes, A model calculation based on the Bogoliubov-de Gennes equations can be used to understand the details of the local tunneling spectra.