The zirconium-rich portion of the Zr−Ga phase diagram was determined by the optical examination of microstructures of isothermally annealed and quenched alloys. A deviation from binary equilibrium, was observed even though careful selection of materials and techniques held impurities to a minimum and produced alloys with a purity of at least 99.9 pct. The slopes of the α-β boundaries are depressed and the range of solubility of the solid solution phases is restricted when compared to the phase diagrams of other Group IIIB elements, apparently as a result of the large difference in atomic size between zirconium and gallium. Thea0 andc0 lattice constants of cph zirconium are contracted and the axial ratio is expanded by the addition of gallium. The change inc/a at 1 at. pct was very close to the change observed in Zr-In alloys, in agreement with general dependence of these properties in zirconium alloys upon electron to atom ratio. A eutectoid reaction occurs at 860°C with β solid solution (1.8 at. pct Ga) decomposing into α solid solution (0.8 at. pct Ga) and Zr3Ga. Cast microstructures suggest a eutectic reaction in which liquid (21.0 at. pct Ga) decomposes into β (8.0 at. pct Ga) and Zr5Ga3. It is proposed that intermediate phases are formed at 25.0 at. pct Ga (Zr3Ga), 37.5 at. pct Ga (Zr5Ga3), and 50.0 at. pct Ga (ZrGa) although the exact composition was not determined.