The influence of neutron irradiation on microstructural evolution and mechanical properties of mono- and polycrystalline molybdenum and its alloys has been investigated. Tensile specimens and 3 mm diameter discs of monocrystals of pure molybdenum and Mo-5%Re were irradiated with fission neutrons at similar to 320 K to displacement doses in the range 5.4 x 10(-4) to 1.6 x 10(-1) dpa (NRT) in the DR-3 reactor at Riso National Laboratory. For comparison, polycrystalline specimens of Mo-5% Re and TZM were also irradiated together with the monocrystalline specimens. Both unirradiated and irradiated specimens were tensile tested at 295 K. Post-irradiation microstructures were quantitatively characterized using a transmission electron microscope (TEM). Fracture surfaces were examined in a scanning electron microscope (SEM). The results of tensile testing as well as of transmission and scanning microscopy are presented and discussed in terms of intracascade clustering of self-interstitial atoms and the role of one-dimensional glide of these clusters in controlling microstructural evolution and the resulting mechanical properties. (C) 1998 Elsevier Science B.V. All rights reserved.