Of 84Staphylococcus aureusstrains isolated from the anterior nares of healthy sheep and from the udders of ewes suffering from purulent mastitis the 89 per cent belonging to the C biotype contained agglutinogenh2as well as polysaccharide Aβ.Ninety‐five per cent of the C biotype strains were lysed by the bovine phage 78, the human phage set giving only weak reactions at RTD X 100. Two pigment‐negative deficit variants as well as three unclassified strains gave similar results while three A biotype strains and one E biotype strain were definitely different. The close correlation between biochemistry, serology and phage typing substantiates the practical usefulness of the subdivision ofS. aureusinto biot