The nucleotide sequences of two gene clusters, cap5 and cap8, involved in the synthesis of Staphylococcus aureus type 5 and type 8 capsular polysaccharides (CPs), respectively were determined. Each gene cluster contained 16 ORFs, which were named cap5A through cap5P for type 5 CP and cap8A through cap8P for type 8 CP. The cap5 and cap8 loci were allelic and were mapped to the SmaI-G fragment in the standard SmaI map of Staph. aureus strain NCTC 8325. The predicted gene products of cap5A through cap5G and cap5L through cap5P are essentially identical to those of cap8A through cap8G and cap8L through cap8P, respectively, with very few amino acid substitutions. Four ORFs located in the central region of each locus are type-specific. A comparison of the predicted amino acid sequences of cap5 and cap8 with sequences found in the databases allowed tentative assignment of functions to 15 of the 16 ORFs. The majority of the capsule genes are likely to be involved in amino sugar synthesis; the remainder are likely to be involved in sugar transfer, capsule chain-length regulation, polymerization and transport.