AbstractÐStudies on saturation behavior produced by cyclic deformation have been conducted on tungsten mono®lament-reinforced monocrystalline copper composites. The eect of the ®ber on strain localization has been investigated using interferometry. For a given applied strain amplitude, local strain and volume fraction of the persistent slip bands (PSBs) in the composite appeared no dierent from those observed in monolithic copper single crystals. However, the distribution of the PSBs was observed to be more uniform, and the total number of PSBs is substantially higher than that in monolithic crystals. The PSBs appeared mostly in the form of micro-PSBs or macro-PSBs with very limited width. Instead of expanding existing PSBs, new PSBs were more likely to nucleate at new locations during cyclic deformation. The volume fraction and width of the PSBs were observed to increase during saturation, which indicates that some of the PSBs become aged and new PSBs form in order to continue to carry the plastic strain. A rule of mixtures model was established to link the cyclic stress±strain response of the monocrystalline composites to the behavior of monolithic single crystals and ®bers. The results calculated from the model show very good agreement with the experimental data.