Highly purified soluble adenovirus type 1 hexons were crystallized by dialyzing against 0.5 M acetate buffer, pH 4.5, for 3 days. Crystals of tetrahedral shape were formed after storage at 4.degree. C within 1-10 days and ranged in size from 0.01-0.1 mm. In the early phase of crystallization, diffusely dispersed hexons were observed by EM. Later, continuous monolayers of 2-dimensional hexon paracrystalline arrays developed. Direct analysis of the electron micrographs and their optical diffraction patterns showed that the structure of the dense 2-dimensional crystalline arrays of hexon capsomers was characterized by slightly skew and irregular hexagonal packing. Multilayered microcrystals with regular linear internal structure were also found. Unique forms of aggregated hexons, such as holey hexon lattice and filamentous forms, were also found.