The penetration of radioactive Na24, Kr85, Rb86, and Xe133 ions with energy between 20 and 100 kev into oriented aluminum single crystals has been measured using the anodic stripping technique. Many of the incident ions penetrated several times farther than normal when the ion beam was aligned with directions of closely packed atoms in the crystal lattice. Comparison with theory showed that these ions traveled down channels between the rows of closely packed aluminum ions. The penetration distance and fraction of incident ions traveling these channels increased successively for beams in the directions , , , and . The energy and mass dependence of the penetration of these channeled ions indicates that an appreciable amount of their energy is lost by electron excitation.