PPIases are ubiquitous in living organisms. While 3 families of PPIases, cyclophilin (CyP), FK506 binding protein (FKBP) and parvulin (Pvn), have been studied in detail in Eukarya and Bacteria (eubacteria), little is known about archaeal PPIases. Among 2 cyclophilins found in Archaea, only Halobacterium cyclophilin (HcCyP19) has been characterized. It is a cyclosporin A (CsA) sensitive CyP with a MW of 19.4kDa. The PPIase activity and CsA sensitivity of this CyP is higher at higher salt concentration in the medium. No parvulin or its homolog has been found in Archaea. Two types of FKBPs, 26-30kDa long type and 17-18 kDa short type FKBP, have been found in Archaea. While the N-terminal regions of these 2 type FKBPs are similar to each other, the long type archaeal FKBP has an additional ca. 100 amino-acid sequence at its C-terminal region. In comparison with human HsFKBP12, the N-terminal region of the archaeal FKBP has 2 insertion sequences in the regions corresponding to Bulge and Flap of HsFKBP12. A short type archaeal FKBP from Methanococcus thermolithotrophicus has been shown to have not only a PPIase activity but also a chperone like activity, which includes protein refolding and aggregation suppressing activities with regard to protein folding intermediates. Mutational analysis revealed that this chaperone-like activity was independent of the PPIase activity, and that the insertion sequence in the region corresponding to the Flap seemed to be important.