The coronavirus spike (S) protein mediates infection of receptor-expressing host cells and is a critical target for antiviral neutralizing antibodies. Angiotensin-converting enzyme 2 (ACE2) is a functional receptor for the coronavirus (severe acute respiratory syndrome (SARS)-CoV) that causes SARS. Here we demonstrate that a 193-amino acid fragment of the S protein (residues 318-510) bound ACE2 more efficiently than did the full S1 domain (residues 12-672). Smaller S protein fragments, expressing residues 327-510 or 318-490, did not detectably bind ACE2. A point mutation at aspartic acid 454 abolished association of the full S1 domain and of the 193-residue fragment with ACE2. The 193-residue fragment blocked S protein-mediated infection with an IC50 of less than 10 nM, whereas the IC 50 of the S1 domain was โˆผ50 ฮผM. These data identify an independently folded receptor-binding domain of the SARS-CoV S protein.