The genome organization of the archaebacteria is investigated in three model systems: a) rRNA genes of various archaebacteria, b) a plasmid of 15.6 kb from Sulfolobus acidocaldarius which exists in free or integrated form, c) the 59 kb genome of phage phi H of Halobacterium halobium as a model for the unusual structural variability of DNA in this organism. Several variants of this phage have been isolated, their genomes differ by several insertions, a deletion, and an inversion. The frequent inversion and circularization of a 12 kb segment of DNA appears to be linked to the presence of two copies of an IS element at its flanks. DNA-dependent RNA polymerases have been isolated from a large number of archaebacteria including representatives of 4 families of the novel order Thermoproteales . As shown by immunological methods, they are closely related to those of eukaryotes. Two different types of RNA polymerase exist in the two main branches of the archaebacteria. The role of one component of the enzyme of Thermoplasma acidophilum was elucidated using an in vitro transcription system.