The taxonomic relationships of seven isolates obtained from porcine stomachs (the "CHY" group), which resembled (but were distinct from) the type strain and other reference strains of Campylobacter hyointestinalis, were examined by using phenotypic and genomic methods. The phenotypic characteristics and ultrastructure of the new organisms were characteristic of Campylobacter species, although they could be distinguished from all previously described taxa. A numerical analysis of 38 phenotypic characters revealed that the new isolates formed a distinct group at a similarity level of 90.1% and could be clearly distinguished from reference strains representing 20 related taxa, principally species and subspecies belonging to the genera Campylobacter, Arcobacter, and Helicobacter. DNA-DNA hybridization studies revealed that the porcine stomach strains were genomically homogeneous (levels of relatedness, 84 to 90%), although the levels of DNA homology with type and reference strains of C. hyointestinalis were relatively high (56 to 71%). Differences in the DNA base compositions of the CHY group and C. hyointestinalis strains were also observed. Our data indicate that the new porcine isolates should be considered members of a subspecies of C. hyointestinalis, for which we propose the name Campylobacter hyointestinalis subsp. lawsonii subsp. nov. The type strain is strain CHY 5 (= LMG 14432 = NCTC 12901 = CCUG 34538). The description of C. hyointestinalis is emended accordingly, and a description of Campylobacter hyointestinalis subsp. hyointestinalis subsp. nov. is given.