Abstract When soluble extracts from the extreme acidophilic archaeon Sulfolobus solfataricus were incubated with [γ‐ 32 P]ATP, several radiolabeled polypeptides were observed following SDS‐PAGE. The most prominent of these migrated with apparent molecular masses of 14, 18, 35, 42, 46, 50, and 79 kDa. Phosphoamino acid analysis revealed that all of the proteins contained phosphoserine, with the exception of the 35‐kDa one, whose protein‐phosphate linkage proved labile to strong acid. The observed pattern of phosphorylation was influenced by the identity of the divalent metal ion cofactor used, Mg 2+ versus Mn 2+ , and the choice of incubation temperature. The 35‐ and 50‐kDa phosphoproteins were purified and their amino‐terminal sequences determined. The former polypeptide's amino‐terminal sequence closely matched a conserved portion of the α‐subunit of succinyl‐CoA synthetase, which forms an acid‐labile phosphohistidyl enzyme intermediate during its catalytic cycle. This identification was confirmed by the ability of succinate or ADP to specifically remove the radiolabel. The 50‐kDa polypeptide's sequence contained a heptapeptide motif, Phe/Pro‐Gly‐Thr‐Asp/Ser‐Gly‐Val/Leu‐Arg, found in a similar position in several hexosephosphate mutases. The catalytic mechanism of these mutases involves formation of a phosphoseryl enzyme intermediate. The identity of p50 as a hexosephosphate mutase was confirmed by (1) the ability of sugars and sugar phosphates to induce removal of the labeled phosphoryl group from the protein, and (2) the ability of [ 32 P]glucose 6‐phosphate to donate its phosphoryl group to the protein.