The problem of crystal structure of the martensitic B1 phase of the eutectic alloy in the Cu-Al system still requires a more accurate clarification (Hun ger and Dienst 1960, Tarora 1949). Martensitic phases in general are formed through small thrust (shear) deformations of the original lattice. On this basis the results of Hunger and Dienst (1960) are doubtful, since the lattice constants found by them have required a considerable reclassification. From the known lattice constants of the B1 phase (Tarora, 1949) and the orientation relationships of the B1 phase (Wassermann, 1934) one has expected a hexagonal lattice for the martensitic phase with [formula].