Fatigue crack growth rates and crack closure have been examined for a body-centered-cubic (bcc) titanium alloy (Ti-30Mo) at five test temperatures ranging from 123 to 340 K. In the same temperature range the influence of internal hydrogen (as provided by gas phase charging) has been studied. Detailed fractographic analyses have been made to quantify the amount of cleavage fracture as a function of test temperature, hydrogen concentration, and stress intensity factor range. The extent of cleavage, both cyclic and static, increased with decreasing temperature. For the lowhydrogen content specimens the fatigue crack growth resistance increased with increasing cleavage over the temperature range from 340 to 190 K. The fatigue crack growth resistance for the high hydrogen alloy remained relatively insensitive to the increasing amounts of cleavage over the same temperature range. An examination of the fatigue crack growth rate data shows that the power exponent in the following expression is in the range of 2 to 2.5 for temperatures of 123 to 340 K: dadn=B(ΔKi−ΔKthi)n where ΔK = ΔKⁱ = ΔK^c and ΔKⁱ is the intrinsic component and ΔK^c is the closure component. These observations indicate that the factor dominating the fatigue crack growth rate and the resulting cyclic cleavage process is the reverse plasticity in the crack tip region. The increased resistance to fatigue crack growth in the temperature range from 340 to 190 K for the low hydrogen contents is attributed to the higher yield stresses in this region. The role of hydrogen in determining fatigue crack growth rates and fatigue thresholds (ΔK_th) is discussed in terms of its influence on both ΔKⁱ and ΔK^c.