Three Arcobacter isolates, recovered from mussels (genus Mytilus), and one isolate from brackish water in Catalonia (north-east Spain) showed a novel pattern using a recently described identification method for members of the genus Arcobacter, 16S rRNA gene RFLP. Enterobacterial repetitive intergenic consensus PCR fingerprinting demonstrated that the three isolates from mussels belonged to two genotypes and that the fourth isolate from water belonged to a third genotype. Analysis of the 16S rRNA and rpoB gene sequences showed that the new isolates formed a separate lineage within the genus Arcobacter. This was also confirmed by the low DNA-DNA relatedness values (16-30 %) of the isolates with the type strains of recognized Arcobacter species. Hydrolysis of indoxyl acetate, a characteristic trait for all species of the genus Arcobacter, was negative for the novel isolates. The susceptibility of the novel isolates to cefoperazone, together with the lack of urease production and nitrate reduction, further enabled them to be differentiated from recognized Arcobacter species based on physiological characteristics. Genotypic and phenotypic characteristics indicated that the new isolates represent a novel species of the genus Arcobacter, for which the name Arcobacter mytili sp. nov. is proposed, with the type strain F2075(T) (=CECT 7386(T) =LMG 24559(T)). The DNA G+C content of strain F2075(T) was 26.9 mol%.