Low angle synchrotron X-ray diffraction patterns were obtained from the demembranated human corneal stroma. These patterns showed differences in the relative intensities of the meridional reflections compared to those obtained using bovine corneas. In particular, the first order reflection, conspicuously absent in the bovine pattern, is present in the human, although it is still only one third as intense as the first order in the pattern from rat tail tendon.Using phases deduced from the electron-optical images of negatively stained corneal collagens, a difference electron density map between bovine and human corneal collagen was drawn. The features in this map are explained in terms of a different inter-relationship between collagen and proteoglycans in the two species. The function of these species differences is unknown.