Staphylococcus aureus is a major pathogen of humans as well as of small and large ruminants. In the two latter, it is a major cause of mastitis which is a hindrance to the dairy industry. Preliminary studies with a few strains have suggested that they present host species-specificity. In order to further investigate this we analysed 157 S. aureus strains, obtained from the nasal cavities and from healthy or infected teats of large (bovine) and small (ovine and caprin) ruminants. The strains were submitted to typing by PFGE and had their accessory gene regulator (agr) group and capsular polysaccharide (cap) type determined. Resistance to the main drugs used to treat mastitis was evaluated by disk-diffusion antibiograms. In spite of the majority of strains being collected in a geographically circumscribed region (north-western France), PFGE pulsotype analysis yielded clusters significantly related to host species (large or small ruminant) independent of site of colonisation/infection. agr group and cap type were also significantly correlated to host species. Levels of resistance to antibiotics, including oxacillin, were low, indicating that domesticated ruminants are not a reservoir of MRSA. This work shows that the nature of S. aureus strains differs between large and small ruminants and suggests the existence of a host- rather than tissue- specificity. Further studies are being pursued for the determination of the molecular nature of this host specificity and strategies for mastitis prevention in small or large ruminants take account of these differences. This work was supported by CAPES-COFECUB project 539/06.