ABSTRACT Staphylococcus aureus is a major human pathogen responsible for increasing the prevalence of community- and hospital-acquired infections. Protein A (SpA) is a key virulence factor of S. aureus and is highly conserved. Sequencing of the variable-number tandem-repeat region of SpA ( spa typing) provides a rapid and reliable method for epidemiological studies. Rarely, non- spa -typeable S. aureus strains are encountered. The reason for this is not known. In this study, we characterized eight non- spa -typeable bacteremia isolates. Sequencing of the entire spa locus was successful for five strains and revealed various mutations of spa , all of which included a deletion of immunoglobulin G binding domain C, in which the upper primer for spa typing is located, while two strains were truly spa negative. This is the first report demonstrating that nontypeability of S. aureus by spa sequencing is due either to mutation or to a true deficiency of spa .