Frequency upconversion has been demonstrated in Er3+- and Yb3+/Er3--doped silica optical fibers pumped by 1064 nm radiation. IR to blue upconversion in Er3+-doped fibers and IR to blue, green and red upconversion in Yb3+/Er3+-doped fibers have been observed. Ground state absorption, excited state absorption, two-photon absorption and energy transfer processes are proposed to account for the experimental observations.