Magnetostrictions of metallic glass ribbons were measured by fiber-optic Mach-Zehnder interferometry. The saturation magnetostrictions measured here are accurate to within 10%. For accurate measurements the fibers in the ribbons were uncoated, the ribbons were flattened before bonding, and two passes of fiber in the sensing arm were bonded to a single layer metallic glass ribbon at the ends only. Various factors affecting the accuracy of the measurements were also discussed.