A fluorescence-based live-cell adhesion assay was used to examine biofilm formation by 20 different haloarchaea, including species of Halobacterium, Haloferax and Halorubrum, as well as novel natural isolates from an Antarctic salt lake. Thirteen of the 20 tested strains significantly adhered (P-value'<'0.05) to a plastic surface. Examination of adherent cell layers on glass surfaces by differential interference contrast, fluorescence and confocal microscopy showed two types of biofilm structures. Carpet-like, multi-layered biofilms containing micro- and macrocolonies (up to 50''m in height) were formed by strains of Halobacterium salinarum and the Antarctic isolate t-ADL strain DL24. The second type of biofilm, characterized by large aggregates of cells adhering to surfaces, was formed by Haloferax volcanii DSM 3757T and Halorubrum lacusprofundi DL28. Staining of the biofilms formed by the strongly adhesive haloarchaeal strains revealed the presence of extracellular polymers, such as eDNA and glycoconjugates, substances previously shown to stabilize bacterial biofilms. For Hbt.'salinarum DSM 3754T and Hfx.'volcanii DSM 3757T, cells adhered within 1 day of culture and remained viable for at least 2 months in mature biofilms. Adherent cells of Hbt.'salinarum DSM 3754T showed several types of cellular appendages that could be involved in the initial attachment. Our results show that biofilm formation occurs in a surprisingly wide variety of haloarchaeal species.