The thermosome of the archaeon Thermoplasma acidophilum is composed of two subunits, alpha and beta, which are arranged in two stacked, eight-membered rings. Electron cryo-microscopy in conjunction with image analysis revealed a 4-fold symmetry in the heterooligomeric alpha + beta thermosome isolated from Thermoplasma, but not in the homooligomeric alpha-only thermosome expressed in Escherichia coli. This indicates that alpha and beta-subunits alternate within the rings of the Thermoplasma thermosome rather than forming two different homooligomeric rings. In addition, a small subpopulation of 9-fold symmetric complexes was found among the recombinant alpha-only thermosomes, and a central mass most likely representing bound substrate molecules was observed in about half of the native and recombinant thermosome particles.