The egg capsules of dogfish are largely constructed from laminae of parallel orientated collagenous fibrils arranged longitudinally, near-diagonally, and near-transversely. The fibrils show an axial (D) period of about 40 nm. These characteristics facilitated the observation of fibril rotation and strain and the construction of an exploratory model of the mechanical behaviour based on composite theory. Low angle X-ray diffraction demonstrated reversible changes in material strained up to 24%; the D period extended by 5% with a loss in transverse order. Electron microscopy confirmed this fibril strain and demonstrated that failure of the interfibrillar matrix at moderate global strains was conspicuous in the near-transverse laminae and less marked in the near-diagonal ones. Light microscopy demonstrated that rotation of the near-diagonal fibrils occurred during global strain. The results of mechanical testing are described. The model helps to explain the double S-shaped stress/strain and unrecoverable deformation plots. The tensile behaviour up to strains of 10% appeared dominated by failure of the interfibrillar matrix in the near-transverse laminae. The behaviour of the material at strains in excess of this was thought to result from a combination of rotation and straining of matrix and fibrils in the near-longitudinal and near-diagonal laminae. The egg capsule is thought to function as an efficient shock absorber reducing the likelihood of injury from both wave action and attempted predation by fish. (C) 1994 Academic Press, Inc.