N-terminal protein acetylation is common in eukaryotes and halophilic archaea, but very rare in bacteria. We demonstrate that some of the most abundant proteins present in the crenarchaeote Sulfolobus solfataricus, including subunits of the thermosome, proteosome and ribosome, are acetylated at the N-terminus. Modification was observed at the N-terminal residues serine, alanine, threonine and methionine-glutamate. A conserved archaeal protein, ssArd1, was cloned and expressed in Escherichia coli, and shown to acetylate the same N-terminal sequences in vitro. The specific activity of ssArd1 is sensitive to protein structure in addition to sequence context. The crenarchaeota and euryarchaeota apparently differ in respect of the frequency of acetylation of Met-Glu termini, which appears much more common in S. solfataricus. This sequence is acetylated by the related Nat3 acetylase in eukarya. ssArd1 thus has a relaxed sequence specificity compared with the eukaryotic N-acetyl transferases, and may represent an ancestral form of the enzyme. This represents another example where archaeal molecular biology resembles that in eukaryotes rather than bacteria.