Vps4 is a AAA ATPase that mediates endosomal membrane protein sorting. It is also a host factor hijacked by a diverse set of clinically important viruses, including HIV and Ebola, to facilitate viral budding. Here we present the three-dimensional structure of the hydrolysis-defective Vps4p(E233Q) mutant. Single-particle analysis, multiangle laser light scattering, and the docking of independently determined atomic models of Vps4 monomers reveal a complex with C6 point symmetry, distinguishing between a range of previously suggested oligomeric states (8-14 subunits). The 3D reconstruction also reveals a tail-to-tail subunit organization between the two rings of the complex and identifies the location of domains critical to complex assembly and interaction with partner proteins. Our refined Vps4 structure is better supported by independent lines of evidence than those previously proposed, and provides insights into the mechanism of endosomal membrane protein sorting and viral envelope budding.