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This paper deals with a study on a perplexing phenomenon which hasbeen recently reported on several titanium alloys which were found not to exhibit athreshold in ambient air at room temperature or 120°C when tests are run at high meanstress levels and very low stress intensity factor ranges. In such conditions, these materialsrevealed a stationary regime with a constant growth rate. This paper is devoted to thedetermination of the factors influencing the occurrence of this phenomenon, includingmean stress level, test frequency (35 Hz and 3.5 Hz), temperature (room temperature,150°C and 500°C) and environment through comparative tests performed in air and in highvacuum.