A low-fibronectin-binding mutant of Staphylococcus aureus strain 879R4SSp generated by transposon Tn918 mutagenesis is attenuated in a rat endocarditis model (J.M. Kuypers & R.A. Procter, 1989, Infect Immun 57, 2306-2312). PCR and Southern hybridization analysis with primers and probes, respectively, for the fnbA and fnbB genes of strains 8325-4 showed that strain 879R4SSp possesses a single fnb gene which is homologous to fnbA. This was confirmed by sequencing 41 bp of 5' non-coding and 237 bp of 5' coding DNA, which showed 97% base identity to fnbA. Southern hybridization and sequencing showed that Tn918 was inserted 41 bp 5' to fnbA in the mutant 879R4SSp/1536, between the promoter and initiation codon. Reduced adherence of the mutant to surface-bound fibronectin correlated with lower expression of a 180 kDa wall-associated fibronectin-binding protein.