Zr 3Al is an ordered fcc alloy of the L1 2 type. The ordered phase appears to be very stable thermally as it has not been possible to disorder Zr 3Al by quenching. It is possible, however, to disorder Zr 3Al by irradiation. In deformed Zr 3Al the superlattice dislocations were observed to be extended, i.e. they consisted of intrinsic stacking faults on {111} bounded by partial dislocations. _ The total Burgers vector is of the type <110> and the superlattice dislocations have dissociated as follows: «110 a ̊ > → 1 3 «211 a ̊ > + 1 3 «12 1 ̄ a ̊ > . During high-temperature annealing of the deformed material, dislocation networks were produced in which all of the dislocation nodes were extended; hence intrinsic and extrinsic stacking faults must alternate at the nodes. Weak beam images were obtained of long and relatively straight dislocation lines in annealed Zr 3Al in order to obtain information on the equilibrium width of the extended dislocations.