It was demonstrated that porous aggregates of (NH4)(3)PW12O40 nanocrystallites were formed by epitaxial self-assembly, when dodecatungstophosphoric acid was precipitated by ammonium hydrogen carbonate in an aqueous solution. Scanning electron microscopy combined with nitrogen and argon adsorption measurements showed that nanocrystallites of (NH4)(3)PW12O40 formed aggregates of ca. 0.3-1 mu m in size, which are porous with a nearly uniform micropore size of 0.6-1.3 nm in diameter, The shape of these aggregates changed from spherical to symmetric dodecahedral as the precipitation temperature increased. Electron diffraction, transmission electron microscopy, and X-ray powder diffraction demonstrated that the nanocrystallites which form the aggregates had the same crystal orientation in each aggregate, leaving pores between the nanocrystallites, and they were connected epitaxially to each other to variable extents, depending on the The dodecahedral aggregates, for which the epitaxial interconnections grow significantly, can be regarded as microporous single crystals. The results were compared with the aggregates of Cs salts which were previously shown to be randomly oriented and exhibit shape-selective catalysis.