The ultrastructure of human astrovirus serotype 2 (H-Ast2) grown in cell culture was analysed by electron microscopy of thin sections and negatively stained preparation. Infected LLCMK2 cells, as visualized in thin sections, contained cytoplasmic aggregates of dense or hollow-cored particles that aggregated in quasicrystalline arrays and were specifically labelled using a rabbit polyclonal anti-Ast2 antiserum. H-Ast2 particles from the supernatant of infected LLCMK2 cells in thin sections after flat- embedding were similar in size to intracellular virions. In negatively stained preparations, these virus particles had an external diameter of 41 nm and exhibited a well defined layer of surface spikes. Pentagonal and hexagonal contours were occasionally visible, and probably correspond to the projections of icosahedral structures. Star-like morphologies and particles with surface triangular hollows were seen in dark areas of the preparations only after a short treatment of the viruses of pH 10. Incubation of the viruses at pH 10.5 induced a rapid disassembly of the virus particles. The finding that the particles with icosahedral geometry and surface spikes are fully infective allows an alternative morphological model to the traditional one for astroviruses to be proposed.