Catastrophic failures of drillpipes are a major concern in HPHT drilling environments. The National Energy Technology U.S. Department of Energy and DNV are investigating mechanisms of crack propagation in ultra-high strength low alloy (UHSLA) martensitic steel drillpipe under cyclic stress as functions of pH H2S concentration and temperature in simulated production fluids. These fluids are deaerated 5%NaCl solution buffered with NaHCO3/Na2CO3 or CO2 in contact with 0.83 kPa H2S. Microscopic investigations of the pre-cracked sample surfaces after corrosion fatigue tests revealed the presence of additional cracks propagated in the bulk material in neutral pH solution at 20°C. This paper discusses effects of the production fluid temperature on susceptibility of the UHSLA steel to environmentally assisted cracking. Post-corrosion fatigue surface characterization methods such as optical and scanning electron microscopies were used in this study.