X-ray diffraction was used to monitor the effects of electron microscope fixation, staining, and embedding procedures on the preservation of the three-dimensional crystalline order in collagen fibrils of rat tail tendon. A procedure is described in which the characteristic 3.8-nm lateral spacing is preserved, with increased contrast, in the diffraction pattern of the embedded fiber. This spacing is correlated with the separation between the tangentially oriented equally spaced lines of density observed in electron microscope ultrathin fibril cross sections of the same material. Optical diffraction of electron micrographs gives an objective measure of the periodicity and suggests that the fibril is composed of concentrically oriented crystalline domains. These observations, when combined with a recent interpretation of the native x-ray diffraction data [Hulmes, D. J. S. & Miller, A. (1979) Nature (London) 282, 878-880] suggest a tentative model for the three-dimensional structure of collagen fibrils.