We describe a new family of repetitive elements in the genome of the archaebacterium Halobacterium volcanii. There are some 20-30 copies of this element, which we designate ISH51. Sequenced copies show typical insertion sequence characteristics (terminal inverted repeats, direct flanking repeats of "target site" DNA). However, members of the ISH51 family are highly heterogeneous, showing on average only 85% primary sequence homology; and some genomic copies appear to be severely truncated. Some ISH51 elements are clustered together in regions of relatively AT-rich DNA. There are at least five such AT-rich "islands" in the H. volcanii genome. Repetitive sequences homologous to ISH51 are found in the genomes of most Halobacterium and Halococcus species.