The genus Halorhabdus, classified in the family Halobacteriaceae, order Halobacteriales in the class Halobacteria, consists of highly pleomorphic, Gram-stain-negative cells. Some strains exhibit a predominance of cocci- or rod-shaped cells. Strains may produce red or nonpigmented colonies. They are aerobic or facultative anaerobic chemoorganotrophs; some strains display very poor growth under aerobic conditions, and fermentative capability may be present. Some can grow only on a very narrow range of organic substrates. All are catalase positive, and some show a positive oxidase reaction. Nitrate is reduced to nitrite. Some strains also reduce nitrite. Halorhabdus spp. are extremely halophilic, and cells quickly lyse in water. Magnesium is either not required or only necessary in very small amounts (0.002 M). They are thermotolerant, with growth in the range 15–57.5°C. Poly-β-hydroxybutyrate (PHB) is produced. The respiratory lipoquinone present is MK-8(VIII-H2); MK-8 is also present in some strains. Different types of diphytanyl ether derivatives are the major polar lipids; phosphatidylglycerol sulfate is always absent. Three species of the genus Halorhabdus have been described and were isolated from a variety of hypersaline environments: sediments in hypersaline lakes (Hrd. utahensis, the type species of the genus), deep-sea brines (Hrd. tiamatea), and salt mine boreholes (Hrd. rudnickae).