Debye-Scherrer photographs have been taken from forty-four slowly cooled and quenched manganese-gallium alloys. These, incorporated with the differential thermal analysis taken at the gallium-rich side, provide a basis for the Mn-Ga constitutional diagram.There are ten different phases beside pure Ga in the entire system. The solid solubility of Mn in Ga is almost undetectable. The α phase is the primary solid solution of Ga in α-Mn. The solubility limit at room temperature is 1.95 at % Ga. The β phase has a homogeneity range from 8.6 to 19.2 at % Ga at room temperature. The structure is that of β-Mn. It may be looked upon as a solid solution of Ga in β-Mn, stabilized at room temperature due to the random substitution of some of the Mn atoms by Ga. The γ phase may be divided into three parts, γ1, γ2, and γ3. γ1 is face-centred cubic, γ2, face-centred tetragonal, while γ3, face-centred tetragonal with a long range order. The structure of γ3 is isomorphous with CuAu I in the Cu-Au system. γ3 is stable at room temperature, the homogeneity range being from 37 to 45 at% Ga; while γ1 is only stable at high temperatures. The transformations from γ1 to γ2 and then to γ3 are of the second degree, the degree of order increasing with the Ga content and decreasing with the temperature. The whole phase γ may be considered as a solid solution of Ga in γ-Mn, which could not be retained by quenching in the pure state. The δ phase exists only at high temperatures. It may be regarded as a solid solution of Ga in δ-Mn, deformed and ordered by the substitution of some of the Mn atoms by Ga. The ε phase has an ordered hexagonal close-packed structure with eight atoms per unit cell. It is formed congruently from the y phase at about 820℃. The homogeneity range at room temperature is estimated to be from 27 to 30 at% Ga. The η phase has a wide range at room temperature, estimated to be from 50 to 60 at% Ga. From 520 to 600℃, it undergoes a polymorphic transformation to another phase λ, the homogeneity range of which being displaced with temperature toward the Mn-rich side. Both η and λ indicate quite complicated structures. At the Ga-rich side, there are three intermediate phases x, φ and ω. They are formed by peritectoid or peritectic reactions. The ideal structure of φ is cubic of the NiHg4 type, but it lies outside of the very narrow homogeneity range around Mn2.3 Gay7.7. Most probably the stoichiometric composition of the ω phase is Mn2Ga9 or MnGa5. Among the six intermediate phases stable at room temperature, β, ε, γ3, x and φ are ferromagnetic. The most pronounced ferromagnetic alloys have been found in the γ3 and φ regions which are rich in Ga content. Saturation magnetizations and Curie points for some of the alloys have been determined.