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This paper is focused on the effect of environment on the fatigue crackpath on two titanium alloys used in aeronautical turbine engines at high temperatures.The objective is to try to put in light the crack propagation mechanisms involvedthrough microscopic observations on the crack path and fracture surface. A Ti-6A-4Valloy for turbine disks is studied at 300°C in three microstrutural states and a Ti6246alloy for turbine disks and blades is used for tests at 500°C, this last alloy beingdedicated to operate at higher temperature. Tests were run in laboratory air, in highvacuum and in other environments with controlled partial pressure of water vapor andoxygen permitting to distinguish the intrinsic behavior from the effects related to theenvironment, the temperature, the stress level which can act separately orsimultaneously. It is shown on this study that the crack path and the fracture surfacemorphology are strongly dependent on the cracking process involved.