The collagen which forms the egg case of the dogfish, Scyliorhinus canicula, is assembled in a three-dimensional network that results in a very resistant capsule. The capsule presumably accomplishes both a protective and a filtering role for the embryo contained within it. In the present work we have obtained electron micrographs of metal-shadowed, deep-etched replicas of the egg ease and we have analyzed the micrographs using computer-based Fourier methods. The replicas provide details of the three-dimensional structure that have not been recorded before, allowing us to add particulars to the [1,0,0], [1,1,0], and [0,0,1] views previously obtained from plastic sections (C. Knupp et al., 1996, J. Struct. Biol. 117, 209-221). A molecular packing arrangement consistent with all of the present data is proposed. Although this collagen type is unique and is substantially shorter (45 nm) than other collagens, investigations into its structure may ave an insight into related collagen types. The role of this collagen as both a protective structure and one with filtering properties controlling permeability is discussed in terms of other collagens with similar functions.