The hyperthermophilic archaea Acidianus hospitalis, Aeropyrum pernix, Pyrobaculum aerophilum, Pyrobaculum calidifontis, and Sulfolobus tokodaii representing three different orders in the phylum Crenarchaeota were analyzed by flow cytometry and combined phase-contrast and epifluorescence microscopy. The overall organization of the cell cycle was found to be similar in all species, with a short prereplicative period and a dominant postreplicative period that accounted for 64 to 77% of the generation time. Thus, in all Crenarchaeota analyzed to date, cell division and initiation of chromosome replication occur in close succession, and a long time interval separates termination of replication from cell division. In Pyrobaculum, chromosome segregation overlapped with or closely followed DNA replication, and further genome separation appeared to occur concomitant with cellular growth. Cell division in P. aerophilum took place without visible constriction.