The low cycle fatigue behaviour of a 9Cr1MoNbV martensitic steel has been investigated at 350 °C in air and inlead–bismuth eutectic (LBE) with low and high oxygen content. Total strain controlled tests were performed from Δεt=0.40% to 1.2%. The material exhibited a pronounced cyclic softening in both environments. LBE reduced the fatigue resistance. LBE accelerated the formation of the long crack by promoting the growth of the first short cracks. A clear change in propagation mode was observed. In air, ductile fatigue striations were observed while in LBE a brittle fracture decorated by voluminous and largely spaced striations were visible. From EBSD analysis, it was concluded that in LBE, the long crack advanced quickly by repeated and discontinuous cleavage.