The application possibilities of the phase contrast method in investigations of both isotropic and anisotropic optical fibers are discussed. In particular, this method enables the core diameter and other geometrical parameters of graded-refractive-index (GRIN) fibers to be determined more accurately when combined with scanning microdensitometry. The refractive index profile of some fibers, especially that of monomode fibers, can be determined by using a negative phase contrast device if the difference Δn between refractive indices of the fiber core and cladding is very small (smaller than 0.03). The result obtained is comparable to that offered by differential microinterferometry. In general, highly sensitive negative phase contrast microscopy, such as that based on the KFA device with soot phase rings (commercially available from the Polish Optical Works, Warsaw), permits us to observe extremely small optical inhomogeneities that cannot be revealed with the help of conventional microinterferometric techniques.