The elasto-optic effect in optical fibers under axial strain can be characterized by means of the whispering gallery mode (WGM) resonances of the fiber itself. This technique enables a direct measurement of the anisotropy, the determination of the individual Pockels' coefficients, and the study of the wavelength dependence. The method is based on a rigorous theoretical study of WGM resonances in cylindrical microresonators. The shift of the WGM resonances as a function of strain was measured for the TE and TM modes, showing a strong modal anisotropy. In particular, the shift rate for TE modes was 1.84 times the one for TM modes. From these measurements, experimental values for the Pockels' coefficients were obtained: p11=0.116 and p12=0.255 at 1531 nm, and p11=0.131 and p12=0.267 at 1064 nm. The dispersion of p44 with wavelength was shown to be 5% μm-1.