Strain typing of microbial pathogens has two major aims: (i) to index genetic microvariation for use in outbreak investigations and (ii) to index genetic macrovariation for use in phylogenetic and population-based analyses. Until now, there has been no clear indication that one genetic marker can efficiently be used for both purposes. Previously, we had shown that DNA sequence analysis of the protein A gene variable repeat region (spa typing) provides a rapid and accurate method to discriminate Staphylococcus aureus outbreak isolates from those deemed epidemiologically unrelated. Here, using the hypothesis that the genetic macrovariation within a low-level recombinogenic species would accurately be characterized by a single-locus marker, we tested whether spa typing could congruently index the extensive genetic variation detected by a whole-genome DNA microarray in a collection of 36 isolates, which was recovered from 10 countries on four continents over a period of four decades, that is representative of the breadth of diversity within S. aureus. Using spa and coa typing, pulsed-field gel electrophoresis (PFGE), and microarray and multilocus enzyme electrophoresis (MLEE) data in molecular epidemiologic and evolutionary analyses, we determined that S. aureus likely has a primarily clonal population structure and that spa typing can singly index genetic variation with 88% direct concordance with the microarray and can correctly assign isolates to phylogenetic lineages. spa typing performed better than MLEE, PFGE, and coa typing in discriminatory power and in the degree of agreement with the microarray at various phylogenetic depths. This study showed that genetic analysis of the repeat region of protein A comprehensively characterizes both micro-and macrovariation in the primarily clonal population structure of S. aureus.Staphylococcus aureus is the leading cause of nosocomial infections (21) and is responsible for a wide range of human diseases, including endocarditis, food poisoning, toxic shock syndrome, septicemia, skin infections, soft tissue infections, and bone infections, as well as bovine and ovine mastitis. The recent emergence of community-associated methicillin-resistant S. aureus (MRSA) strains (23) and the movement of the vanA operon from Enterococcus faecalis into MRSA, establishing strains with high-level vancomycin resistance (3), further heighten the public health concerns. Thus, understanding and controlling the spread of S. aureus, in both the hospital and community settings, is now of paramount importance.S. aureus is a heterogenous (polymorphic) species (13) that was recently found to have a clonal population structure (11). Therefore, it is believed that S. aureus does not undergo extensive recombination, diversifies largely by nucleotide mutations, and shows a high degree of linkage disequilibrium (nonrandom associations between genetic loci). In order to distinguish strains within a heterogenous species for local epidemiologic or outbreak investigation purposes, a highly discriminating ge...