The susceptibility of the ovine mammary gland to experimental infection with Staphylococcus aureus subsp anaerobius was studied in five lactating Churra ewes. The left mammary gland of each ewe was inoculated with bacteria while the right halves remained untreated and served as a control. A rapid cellular response was elicited in all inoculated glands. The reaction of inoculated mammary glands varied from that of clinical mastitis (ewes C and D) to subclinical mastitis (ewes B and E) or merely to a transient increase of somatic cell counts (ewe A). S aureus subsp anaerobius is less pathogenic for ewes by the intramammary route than S aureus since doses of about 108 bacteria caused clinical mastitis in only two of the inoculated animals. © 1993.