The bioceramic shell of the Eastern oyster was investigated to determine its physical and chemical compositions. Resistance and electrolyte conductivity measurements through the shell using a four-electrode cell configuration were made. The scanning electron microscopy/energy-dispersive spectroscopy analysis of oyster shell cross sections revealed that this multilayered biocomposite material is composed of numerous incongruent "chalky" and "nonchalky" layers, each containing similar chemical elements but having different overall chemical compositions and porosities, with the chalky layers being quite porous and voluminous, whereas the nonchalky layers appear to be nonporous, more dense, and compact. The electrolyte conductivity within the pores of the shell was dependent on the ionic concentration of the electrolyte, suggesting that the tortuosity across the shell material via the various chalky and nonchalky layers has an effect on the transport of ions. Rather, consideration of the data in terms of the space charge model indicates that the surface charges within the porous biocomposite facilitate the conduction of ions, much like a ceramic membrane. These findings suggest that the oyster shell may function not only as a protective physical structure, but also as a ceramic membrane allowing for the exchange of ions from the external environment. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3230623] All rights reserved.