Previous studies have shown that 13 genes located in a 14.6-kb region of the chromosome of Staphylococcus aureus (Sa) M are required for type-1 capsular polysaccharide (CP1) biosynthesis. In this report, a total of 17 Sa strains producing different CP serotypes were analyzed by Southern hybridization using DNA probes from the cap1 coding region and the flanking sequences. The results showed that the sequence encoding cap1 genes was specific to CP1-producing strains. In addition, DNA regions of at least 18 kb flanking the cap1 genes were absent in most of the non-type-1 strains. These data suggest that the cap1 genes are associated with a chromosomally located discrete genetic element. One end of the element, referred to as the cap1 element, is located in a 1.7-kb fragment about 11.1 kb upstream from the first gene of the cap1 locus and the other end is located in a 0.8-kb region about 7.6 kb downstream from the last gene of the cap1 locus. Thus, the size of the cap1 element is between 33.3 and 35.8 kb.