The haloarchaea are a group of extremely halophilic archaea that live in salt lakes and other hypersaline environments around the world. They are known to exist with numerous viruses that can infect and kill them, and have developed elegant defence mechanisms, including CRISPR-Cas systems, in order to survive. Such systems also protect against invasion by other foreign DNAs, such as plasmids. CRISPR-Cas systems confer adaptive immunity to the cell, and act by accumulating short spacer sequences (34-39 nt) derived from invading DNAs that can be used to recognize and degrade invading nucleic acids with identical or near-identical sequence. A well-studied model haloarchaeon that can be genetically manipulated is Haloferax volcanii, originally isolated from the Dead Sea, which possesses a single CRISPR-Cas system belonging to type I-B. It has 8 Cas proteins and 3 spacer arrays. In an experimental system to study CRISPR-Cas specificity and function in this organism, it was found that Cas6b is essential for crRNA maturation but not required for the defence reaction (1). Also, six protospacer adjacent motif (PAM) sequences are recognised by the Haloferax defence system, and successful invader recognition requires a non-contiguous seed sequence of 10 base-pairs between the crRNA and the invader. We also examined the spacer sequences carried by this organism to determine if they were related to known haloviruses and plasmids, and found specific relationships to novel mobile elements and viruses (denoted Hvol-IV1 and HFIV1).