Low temperature leucite is known to have a tetragonal structure which is similar to the high temperature cubic structure. This tetragonal structure has never been studied because of the fine twinning, which makes the selection of untwinned single crystals difficult. The best crystal chosen for this research is made up of three individuals: one, comprising 79 per cent of the total, is merohedrically twinned with respect to the second, and pseudo-merohedrically with respect to the third. The crystal structure of tetragonal leucite, a = 13.09, c = 13.75 Å, space group I41/a, can be derived by displacing the cubic structure (some atoms must be shifted as much as 1 Å). There is no (Si,Al) order. The K atoms are coordinated by six oxygens at the average distance of 3.01 Å, six other being at ~3.7 Å. In cubic leucite these values are 3.35 and 3.54 Å; this is the most striking difference between the crystal structures.