The phosphoenolpyruvate synthase of the hyperthermophilic archaeon Staphylothermus marinus forms an unusually large homomultimeric complex of 93 kDa subunits. Electron image analysis of negatively stained and low-dose unstained preparations showed that the complex has a single, stable characteristic view and a well-preserved core with threefold rotational symmetry. The periphery of the assembly is composed of a nebulous, possibly flexible, component. Mass measurements by scanning transmission electron microscopy yielded a molecular weight of 2250 +/-230 kDa, confirming the well-defined nature of the structure and indicating that it is composed of 24 +/- 2.5 subunits. The stability and symmetry of the characteristic projection views suggest a polyhedral three-dimensional architecture. The novel quaternary arrangement of this enzyme might be a consequence of its adaptation to an extreme environment. (C) 1996 Academic Press, Inc.