Transmission electron microscopy has been used to study the effect of ion implantation of krypton ions into cobalt. This has been found to result in the formation. of a high concentration of bubbles with those having diameters below ≈ 3nm being solid at room temperature. Electron diffraction has been used to characterise these solid bubbles or precipitates and this has shown them to possess a hcp crystalline structure and to exhibit an epitaxial relationship with the matrix. Bubble ordering into rafts parallel to the basal plane of the metal has also been observed although the bubble position within the rafts appeared random. These results extend the data available on inert gas precipitation in hcp metals.