Phage ϕH is a temperate phage, i.e., it can establish lysogeny in the archaebacterium Halobacterium halobium. ϕH-lysogens are immune to phage infection and phage production is spontaneously induced at a rate of about 10-7. In the prophage state. ϕH DNA exists as a covalently closed circle of 57 kb. The frequent occurrence of clones carrying the phage genome but unable to produce phage is another proof of the high variability of DNA in H. halobium. In one such strain, R1-3, the phage genome has undergone a structural change which may have abolished an essential phage gene.