X-ray diffraction of hydrated fibrillin rich microfibrils, in the form of zonular filaments from bovine eyes, demonstrated meridional diffraction peaks indexing on a fundamental periodicity of similar to 56 nm in the relaxed state. The effect of sample extensions of up to 50% in length produced an increase in the axial periodicity of only 4% as judged by alteration of the diffraction peak position of the third meridional order. This effect was shown to be reversible. Further extension to 100% of the tissue rest length caused extensive deterioration in the quality of the diffraction and resulted in a more compiler meridional diffraction series, where the fundamental axial periodicity also changed to a length of approximately 80 nm. The fibrillin diffraction image also contains an equatorial diffraction peak that is enhanced upon tissue extension. The measurement of the molecular spacing from the equatorial diffraction profile indicated that the closest approach of molecules gave a broad interference peak of spacing 28 nm, this is nearly twice the molecular diameter as estimated from electron microscopy of dehydrated samples.