The moderately halophilic bacterium Chromohalobacter marismortui contains a 17.5-kb narrow-host-range plasmid, pCM1, which shows interesting properties for the development of cloning vectors for the genetic manipulation of this important group of extremophiles. Plasmid pCM1 can stably replicate and is maintained in most gram-negative moderate halophiles tested. The replication origin has been identified and sequenced, and the minimal pCM1 replicon has been localized to a 1,600-bp region which includes two functionally discrete regions, the oriV region and the repA gene. oriV, located on a 700-bp fragment, contains four iterons 20 bp in length adjacent to a DnaA box that is dispensable but required for efficient replication of pCM1, and it requires trans-acting functions. The repA gene, which encodes a replication protein of 289 residues, is similar to the replication proteins of other gram-negative bacteria.