A Gram-negative bacterium, designated LA31B(T), was isolated from water collected from a hypersaline lagoon on Laysan Atoll in the north-western Hawaiian Islands. Single cells of LA31B(T) were slightly curved but became helical as their length increased. Preliminary characterization based on 16S rRNA gene sequence analysis showed that LA31B(T) shared 96.0 % identity with an Arcobacter sp. isolated from a cyanobacterial mat in hypersaline Lake Sinai, and 94 % identity with Arcobacter nitrofigilis, the type species of the genus Arcobacter. A polyphasic taxonomic study was conducted and confirmed the phylogenetic affiliation of strain LA31B(T) to the genus Arcobacter. However, LA31B(T) was found to be distinct from all recognized Arcobacter species, by a comprehensive biochemical test analysis, whole-cell fatty acid profiling, DNA G + C content (35 mol% in LA31B(T)) and degree of DNA-DNA reassociation. Most notably, LA31B(T) was found to be an obligate halophile, a hitherto undescribed feature among recognized Arcobacter species. These data indicate that LA31B(T) should be considered to represent a novel species in the genus Arcobacter, for which the name Arcobacter halophilus sp. nov. is proposed. This is the first obligately halophilic member of the genus. The type strain is LA31B(T) (=ATCC BAA-1022(T) = CIP 108450(T)).