The initiations of slip, fracture and rupture in tension were studied in sintered and cast molybdenum specimens in the temperature range 77–474°K using optical and scanning electron microscopy. The ductile—brittle transition temperatures, T T , were ~212°K for the sintered material and ~160°K for both coarse and fine-grained cast material. For all batches, at all test temperatures, slip lines were first observed at stresses between 0.35 and 0.50 of the yield stress (in tension, σ Y , or compression, σ Y c ). In cast materials brittle fracture took place at stresses equal to σ Y c , but in the sintered specimens at any stress between the microyield stress, σ M , and σ Y c . Slip lines crossing grain boundaries, cross-slip, slip on more than one system were observed in ductile and brittle, sintered and cast, tensile specimens strained to stresses between σ M and σ Y c . Non-propagating surface microcracks were detected only in ductile specimens at stresses greater than σ Y and only in one sintered specimen at a stress lower than σ Y . The majority of these cracks were intergranular and approximately normal to the tensile axis. In sintered material they were more numerous and were nucleated concurrently with increasing plastic deformation (up to ~30 per cent nominal strain). Although crack nucleation was predominantly intergranular, crack propagation was mainly transgranular in cast specimens and both transgranular and intergranular in sintered specimens. Above the fracture mode transition temperature, in sintered material, voids were formed predominantly at grain boundaries and rupture took place by the pulling out of the fibre-like grains and the shear of the inter-void fibres.