The aim was to determine the evolutionary position of the Staphylococcus aureus clonal complex 75 (CC75) that is prevalent in tropical northern Australia. Sequencing of gap, rpoB, sodA, tuf, and hsp60 and the multilocus sequence typing loci revealed a clear separation between conventional S. aureus and CC75 and significant diversity within CC75.To date, more than 50 species and subspecies of the genus Staphylococcus have been described (5, 13). The most prominent species, Staphylococcus aureus, is a major human pathogen that can cause a wide variety of hospital-and communityacquired infections (9). Strains carrying the mobile genetic element SCCmec (staphylococcal cassette chromosome mec) are resistant to a broad range of ␤-lactam antibiotics and are termed methicillin (meticillin)-resistant S. aureus (MRSA).Evolutionary relationships within the genus Staphylococcus have previously been determined using homologous housekeeping genes (3,6,7,14,15,20), and S. aureus has been shown to be well separated from the other species. Multilocus sequence typing (MLST) of large numbers of S. aureus isolates has revealed that the MLST loci are in general no more than 3% divergent (4). This is considerably less than in other bacterial species (2, 8). Therefore, the picture that has emerged is that extant S. aureus bacteria are descended from a relatively recent common ancestor and that there is a wide evolutionary gulf between S. aureus and any other bacterial species.Recent evidence indicates that this model is an oversimplification. Surveillance of staphylococcal carriage and disease in the tropical northern region of the Northern Territory of Australia yielded methicillin-susceptible and methicillin-resistant Staphylococcus isolates that are highly divergent from S. aureus at the MLST loci and are therefore difficult to classify. These isolates were shown to be closely related to each other and were designated clonal complex 75 (CC75) (16). They accounted for 23% of all isolates and 70% of MRSA isolates recovered during a community study of impetigo (16), and clinical isolates were associated with a variety of disease states (22). Similar to other incidents of community-acquired MRSA,