Ion implantation of pure aluminium with thallium induces formation of nanosized crystalline thallium inclusions with either fee or bee structure. The size of the inclusions depends on the implantation conditions and subsequent annealing treatments. Inclusions less than 10-15 nm in size are generally fee while larger inclusions are bee. The fee inclusions are aligned topotactically with the aluminium matrix with a cube/cube orientation relationship, and they have a truncated octahedral shape bounded by {111} and {001} planes. The lattice parameter of the fee thallium inclusions is 0.484 nm ± 0.002 nm, which is slightly but significantly larger than for the high pressure fee thallium phase known to be stable above 3.8 GPa. The lattice parameter of the bec inclusions is close to the equilibrium value of 0.387 nm and the orientation relationship is given by the Kurdjumov-Sachs rule (011)bcc ║ (111)fcc and [111]bcc ║ [101]fcc The bec inclusions,, representing the high temperature equilibrium phase but existing in metastable equilibrium at lower temperatures, have curved and less well-defined facets. Inclusions with hep structure, the stable phase at room temperature, have not been observed.