Atomic force microscopy has been used to probe the surface nanostructures of severe acute respiratory syndrome coronavirus (SARS-CoV). Single crown-like virion was directly visualized and quantitative measurements of the dimensions for the structural proteins were provided. A corona of large, distinctive spikes in the envelope was measured after treatment with hydroxyoctanoic acid. High-resolution images revealed that the surface of each single SARS-CoV was surrounded with at least 15 spherical spikes having a diameter of 7.29 +/- 0.73 nm, which is in close agreement with that of S glycoproteins earlier predicted through the genomes of SARS-CoV. This study represents the first direct characterization of the surface ultrastructures of SARS-CoV particles at the nanometre scale and offers new prospects for mapping viral surface properties.