The fracture toughness of nuclear-grade hot-pressed beryllium after irradiation to fluences of 3.5 to 5.0 × 10²¹ neutrons (n)/cm² (E > 1 MeV) was determined. Procedures and data relating to a round-robin test contributing to a standard ASTM method for unirradiated beryllium are discussed in connection with the testing of irradiated specimens. A porous grade of beryllium was also irradiated and tested, thereby enabling some discrimination between the models for describing the fracture toughness behavior of porous beryllium. The fracture toughness of unirradiated 2 percent beryllium-oxygen nuclear-grade beryllium was 12.0 MPa m^1/2, which was reduced 60 percent upon irradiation at 339 K and testing at 295 K. The fracture toughness of a porous grade of beryllium was 13.1 MPa m^1/2, which was reduced 68 percent upon irradiation and testing at the same conditions. Reasons for the reduction in fracture toughness upon irradiation are discussed.