The crystal structure of the γ3 phase in the Mn-Ga system has been determined by the Debye-Scherrer method, and thence the degree of order in the atomic arrangement in this phase region has been estimated as a function of the gallium content.The structure of the γ3 phase belongs to the tetragonal system, the space group being D4h1-P4/mmm. The ideal stoichiometric composition is MnGa, and each unit cell contains one formula weight, the manganese and gallium atom occupying (000) and 1/2 1/2 1/2) positions respectively. It is a distorted form of the B structure, and if it is rotated π/4 about the c-axis, the transformed unit cell can be shown to be of the type LI0, isomorphous with CuAu-I. At room temperature, the lattice spaeings of the original cell at 42.9 at.% Ga are α=2.7475? and c=3.6756?.This phase may be considered as a substitutional solid solution of γ-Mn, stabilized at room temperature due to the partial substitution of manganese atoms by gallium atoms as a tetragonal face-centred ordered structure. In this entire phase region, the degree of order increases with the gallium content.