In several euryarchaeota, phosphoglucose isomerase (PGI) activity is catalyzed by an enzyme unrelated to the well-known family of PGI enzymes found in prokaryotes, eukaryotes and some archaea. In order to understand the mechanistic differences between the two families of enzymes we have crystallized PGI from the archaeon Pyrococcus furiosus. The crystals belong to the space group P2(1) and a complete dataset extending to 1.9 A resolution has been collected.