Glyceraldehyde-3-phosphate dehydrogenase in the archaeon Sulfolobus solfataricus (SsGAPD) has been purified 232 fold with an overall recovery of about 25%. The enzyme is a homomeric tetramer with an M(r) of 41 kDa/subunit. It utilizes either NAD+ or NADP+ as coenzyme but its affinity for the latter is about 50 fold higher. SsGAPD activity is maximum at 87 degrees C. In the range 45-87 degrees C the Arrhenius plot is linear and the activation energy is 55 kJ/mol. The enzyme is thermostable, with a half-life of 45 min at 87 degrees C. The primary structure of SsGAPD shows 35% identity with that of other archaeal GAPDs. Its N-domain shows sequence motifs typical of the dinucleotide binding proteins while the catalytic C-terminal region contains a cysteine residue (C140), required for catalysis, that is conserved in all the archaeal, eukaryal and bacterial GAPDs. These remarks suggest that archaeal GAPDs show a convergent molecular evolution to the eukaryal and eubacterial enzymes in the catalytic region.