The most common measures of ductility are uniform and total elongation and reduction of area. Pure copper in the unirradiated state exhibits large amounts of both measures of ductility along with a serpentine glide fracture morphology. After irradiation at 411 to 414°C with fast neutrons to 34 or 50 displacements per atom (dpa), the tensile and fracture behaviors change greatly. Significant uniform elongation is retained, but the reduction of area is very small. Such a unilateral shift between macroscopic measures of ductility is unusual. The fracture surface is also unusual and reflects not only the influence of the large swelling levels attained during irradiation but also the distribution of swelling near grain boundaries. The unique fracture mode in highly voided copper appears to enhance susceptibility to crack propagation and sudden failure without necking, even though the material exhibits a significant level of uniform elongation prior to failure.