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Fibers doped with rare-earth constituents such as Er3+ and Yb3+ are exceedingly important to designers of fiber-optical systems due to their ability to amplify signals in the near infra-red, low-absorption regions of conventional silicate fibers. Extending the range of operating conditions for these systems to include adverse radiation environments requires a detailed study of the behavior of the fiber when subjected to relevant radiation fluxes of various cumulative doses and dose rates. Of particular interest in many applications is the effect of gamma radiation, which is known to degrade optical signal transmittance by creating absorption centers in the material. A study of radiation-induced photodarkening effect in unpumped Er3+, Yb3+, and Er3+/Yb3+ co-doped fibers under Co60 gamma-irradiation is the focus of this paper. Specifically, the temporal evolution of the fiber transmittance in the near infra-red region from ~1.0 μm-1.6 μm was investigated, subjected to a multitude of exposure conditions spanning different dose-rates and total accumulated doses. The Er3+/Yb3+ co-doped fiber was found to be the most radiation resistant, while the Er3+ doped fiber was found to be the most radiation sensitive in this wavelength region. Dose rate and compositional dependencies were also observed in all fibers.