We report on the first step in mapping out the spatial location of structural proteins within the exosporium, namely a description of its three-dimensional architecture. Using electron microscopy and image analysis, we have characterized crystalline fragments from the exosporium of Bacillus cereus, B. thuringiensis and B. anthracis strains and identified up to three distinct crystal types. Type I and type II crystals were examined in three dimensions and shown to form arrays of interlinked crown-like structures each enclosing a cavity approximately 26-34 A deep with threefold symmetry. The arrays appear to be permeated by tunnels allowing access from one surface to the other, possibly indicating that the exosporium forms a semi-permeable barrier. The pore size of approximately 23-34 A would allow passage of the endospore germinants, alanine or inosine but not degradative enzymes or antibodies. Thus the structures appear compatible with a protective role for the exosporium. Furthermore the outermost crystalline layer must act as a scaffold for binding the BclA protein that contributes to the 'hairy nap' layer. The array of crowns may also act as a matrix for the binding or adsorption of other proteins that have been identified in the exosporium such as GroEL, immune inhibitor A and arginase.