In the viviparous bonnethead shark, Sphyrna tiburo, a fluid-filled, acellular egg capsule surrounds fertilized eggs and developing embryos throughout gestation. Like other placental shark species, the capsule remains intact even at the placental implantation site. Although its intervention between the uterine and embryonic tissues of the placenta has long been thought to mediate physiological exchange, little information is available concerning even its basic structure or permeability to solutes. The 1 mum thick capsule wall consists of an inner layer of gelatinous material and an outer layer consisting of at least three laminae of orthogonally arranged fibrous material. These fibers are irregular and often branched. Permeability experiments showed that solutes less than 1,355 Da diffuse across the egg capsule whereas those greater than 6,000 Da do not pass through the membrane. Solute movement across the capsule is a concentration-dependent phenomenon indicating diffusion rather than active transport. Experimental data also suggest that there is an increase in the permeability of the egg capsule to low molecular weight materials during mid- and late gestation. These observations are discussed in relation to the function of the egg capsule as a mediator of maternal-embryonic interactions in matrotrophic sharks.