Thirteen strains of Bdellovibrio were isolated from sea water off the coast of Oahu, Hawaii. Electron microscopic examination of the isolates indicated that all had a single, sheathed, polar flagellum with a diameter of 24–27 nm. Determination of host range on species of marine bacteria (Beneckea, Photobacterium, Alteromonas, Pseudomonas, and Alcaligenes) as well as terrestrial species (Escherichia, Salmonella, Aerobacter, Aeromonas, Vibrio, and Pseudomonas) indicated that the marine bdellovibrios were able to parasitize 17–32 species out of a total of 42 tested. In general, bacteria of marine origin were better hosts than terrestrial strains; Vibrio cholerae, Aeromonas formicans, and Salmonella typhimurium were, however, lysed by some strains of Bdellovibrio at a rate and intensity comparable to the marine hosts. A numerical analysis of the host range indicated that on the basis of this property the 13 strains of Bdellovibrio could be subdivided into three groups. Three HI strains were isolated from one group and were found to have a moles % GC content in their DNA of 38.6 moles %, different from that of any described terrestrial species. The moles % GC content of a HI strain from another group was 43.5. All of the marine isolates required 75–100 mM sodium chloride for plaque formation and the optimum was 125–150 mM. The requirement for sodium could not be replaced by potassium. HI strains grew only in media in which sodium was present and failed to grow in media in which this ion was replaced by equimolar amounts of potassium. Enumeration studies indicated that the coastal waters of Oahu, Hawaii, had about 121–194 bdellovibrios per liter of sea water. The results of this study suggested that the marine bdellovibrios differed from terrestrial isolates in a number of properties which were a reflection of their marine habitat.