In titanium alloys, dwell periods during room-temperature stress-controlled fatigue tests are responsible for substantial reductions in lifetime compared to pure fatigue loading. The mechanisms of such a creep-fatigue interaction have been investigated for alloy Ti6242. SEM and TEM observations revealed crack initiation by coalescence of shear-induced cavities nucleated at a/b interfaces in large colonies of a laths nearly parallel to the loading axis. The density and average size of cavities were larger in dwell-fatigue and creep than in fatigue. A qualitative micromechanical model of cavity nucleation based on discrete dislocation dynamics was developed. The number of cycles for cavity nucleation was computed as a function of the applied stress range. A finite threshold, dependent on the size of a laths colonies with similar orientation, was found. The simulations predict earlier cavity nucleation in creep or dwell-fatigue than in pure fatigue, which is consistent with the performed experiments.