One of the main challenges in nanoscience and nanotechnology consists in the production and isolation of metallic atomic-scale nanowires (Benzryadin, C. N.; Lau, A.; Tinkham, M. Q. Nature 2000, 404, 971-974; Zach, M. P.; Ng, K. H.; Penner, R. M. Science 2000, 290, 2120-2123; Nilius, N.; Wallis, T. M.; Ho, W. Science 2002, 297, 1853-1856.). Here we report a unique and controllable way of isolating individual atomic molybdenum (Mo) chains by their encapsulation inside double-walled carbon nanotubes, exhibiting inner diameters ranging from 0.6 to 0.8 nm. We have found that these individual atomic chains form spontaneously within the hollow core of tubes in the absence of any reducing agent. We believe that these atomic-scale nanowires could now be studied without suffering oxidation, so that their physical and chemical properties are elucidated.