The field theory of guided waves in optical fibers with step-index profiles and in which both core and cladding
are chiral isotropic media is developed. We show that both surface and semileaky modes can propagate in
optically active fibers. To shed light on the guidance and leakage properties of chiral isotropic fibers we
present a physical interpretation and several numerical results. The new leakage effect associated with semileaky
modes is an important property that cannot be neglected in the analysis of chiral optical fibers but that,
nevertheless, has been systematically disregarded in the literature.