A method to determine the individual strain-optic coefficients in single-mode fibers is described. It is based on two photoelastic experiments, namely, the polarimetric measurement of optical activity induced by mechanical twist and the interferometric measurement of optical-path-length change induced by static longitudinal strain. For fibers with pure silica core and B//2O//3 doped cladding, the optical activity per unit twist rate and the phase change per unit fiber elongation have been measured to be 0. 1472 and 1. 150 multiplied by 10**7 rad/m, respectively. The strain-optic coefficients have been measured to be p//1//1 equals 0. 113 and p//1//2 equals 0. 252, 7% lower than those of bulk silica.