Chaperonins are multisubunit double-ring complexes that mediate the folding of nascent proteins  . In bacteria, chaperonins are homo-oligomeric and are composed of seven-membered rings. Eukaryotic and most archaeal chaperonin rings are eight-membered and exhibit varying degrees of hetero-oligomerism  . We have cloned and sequenced seven new genes encoding chaperonin subunits from the crenarchaeotes Sulfolobus solfataricus, S. acidocaldarius, S. shibatae and Desulfurococcus mobilis. Although some archaeal genomes possess a single chaperonin gene, most have two. We describe a third chaperonin-encoding gene (TF55-gamma) from two Sulfolobus species; phylogenetic analyses indicate that the gene duplication producing TF55-gamma occurred within crenarchaeal evolution. The presence of TF55-gamma in Sulfolobus correlates with their unique nine-membered chaperonin rings. Duplicate genes (paralogs) for chaperonins within archaeal genomes very often resemble each other more than they resemble chaperonin genes from other archaea. Our phylogenetic analyses suggest multiple independent gene duplications - at least seven among the archaea examined. The persistence of paralogous genes for chaperonin subunits in multiple archaeal lineages may involve a process of co-evolution, where chaperonin subunit heterogeneity changes independently of selection on function.