RNAIII, a 514-nt RNA molecule, regulates the expression of many Staphylococcus aureus genes encoding exoproteins and cell-wall-associated proteins. We have studied the structure of RNAIII in solution, using a combination of chemical and enzymatic probes. A model of the secondary structure was derived from experimental data with the help of computer simulation of RNA folding. The model contains 14 hairpin structures connected by unpaired nucleotides. The data also point to three helices formed by distant nucleotides that close off structural domains. This model was generally compatible with the results of in vivo probing experiments with dimethylsulfate in late exponential-phase cultures. Toe-printing experiments revealed that the ribosome binding site of hld, which is encoded by RNAIII, was accessible to the Escherichia coli 30S ribosomal subunit, suggesting that the in vitro structure represented a translatable form of RNAIII. We also found that, within the 3' end of RNAIII, the conserved hairpin 13 and the terminator form an intrinsic structural domain that exerts specific regulatory activity on protein A gene expression.