During a Danish study on the prevalence of campylobacteria in pig abortions and food of animal origin, eight Gram-negative, slightly curved, rod-shaped, non-spore-forming bacteria were clustered by using amplified fragment length polymorphism analysis in a distinct phenon within the genus Arcobacter. In the present study, numerical analysis of whole-cell protein profiles also showed that all isolates clustered in a single group distinct from other recognized Arcobacter species. DNA-DNA hybridization among two representative strains exhibited a mean DNA-DNA relatedness value of 79 %. DNA-DNA hybridization with the type strains of recognized Arcobacter species revealed levels of DNA-DNA relatedness of 41 % or less. The DNA G+C content of the type strain was 28.5 mol%. Pairwise comparison of the 16S rRNA gene sequences with those of the type strains of established species identified Arcobacter cryaerophilus (97.9 %), Arcobacter cibarius (97.5 %) and Arcobacter skirrowii (97.2 %) as the nearest phylogenetic neighbours. The isolates could be distinguished from other Arcobacter species by means of the following biochemical tests: activities of catalase and urease, reduction of nitrate and growth on minimal medium, lack of growth at 37 degrees C under standardized aerobic and microaerobic conditions, in 4 % NaCl and 1 % glycine media. Finally, DNA fingerprints obtained by using enterobacterial repetitive intergenic consenus-PCR showed that the eight isolates represent eight strains of a single novel Arcobacter species, for which the name Arcobacter thereius sp. nov. is proposed. The type strain is LMG 24486(T) (=CCUG 56902(T)).