Several phenotypic variants ofHalobacterium halobiumarise spontaneously at extremely high frequencies (up to 1%) and are readily identified by inspection of bacterial colonies. Two mutant types, those lacking the buoyant gas vesicles or the photosynthetic purple membrane, have been studied in detail by phenotypic and molecular genetic analysis. In the wild-type NRC-1 strain, thebopgene, encoding the purple membrane protein bacterio-opsin, is found on the bacterial chromosome, while the gas vesicle protein genes,gvpAandgvpC, are present on pNRC100, a multicopy plasmid of ~150 kilobase pairs. ThegvpAandgvpCgenes are on a single transcription unit, while the majorbopmRNA is monocistronic. Essentially all of the purple membrane deficient mutants contain insertion sequence (IS) elements into or upstream of thebopgene. Two elements, ISH 1 and ISH 2, account for most (80–90%) of the purple membrane mutations, but at least three other elements, ISH 23, ISH 26, and ISH S1, have also been implicated. The gas vesicle mutants are more heterogeneous, with many displaying partial phenotypes. Three major classes of gas vesicle mutants are distinguishable: class I and class III mutants are the result of large deletions in pNCR100; however, while class I mutants are partially gas vesicle deficient and contain a correspondingly reduced number ofgvpACoperon copies, class III mutants contain no detectable copies of the gas vesicle genes and are essentially completely gas vesicle deficient. Class II mutants, like the purple membrane mutants, contain IS elements into or upstream of the gas vesicle genes. At least five different IS elements are involved: ISH 2, ISH 3, ISH 4, ISH 6, and ISH 8.Key words: insertion sequence, transposable element, plasmid,Halobacterium halobium, bacteriorhodopsin, gas vesicle.