PtGa[sub 2] has previously been reported to exist as both a brass-colored, fluorite-structured phase and as a silver-colored alloy of unknown crystal structure. The crystal structure of the silver- colored form is reported here, along with a discussion of the stoichiometric factors responsible for the polymorphy of this phase. The silver- colored form belongs to the space group I4[SUB 1]/acd, with the lattice parameters a = 8.5544(4) and c = 21.574(17) Angstroms, and Z = 32. The structure consists of layers of Pt and Ga stacked along the c axis, in which two crystallographically different Pt and Ga atoms have similar environments involving eight and nine nearest neighbors, respectively. This structural arrangement may be the prototype for a family of ternary platinum metal-Group B-based phases. The present investigation also necessitates changes in the currently accepted Ga- Pt phase diagram. Stoichiometric PtGa[sub 2] undergoes a structural transformation upon cooling from a brass-colored fluorite structure to this silver-colored, tetragonal structure. However, the transformation is inhibited if the composition of the fluorite- structured phase is Ga-poor
