Experiments designed to study the movement of genes between the plasmids and the chromosome of Staphylococcus aureus are described.Two derivatives of the plasmid pI9789::Tn552, that are temperature-sensitive for replication, pSl and pAD, are used as a model system for this investigation. The plasmids carry a gene conferring resistance to cadmium salts and so the number of cadmium-resistant colonies appearing on selective plates at a restrictive temperature is used as a means of measuring the frequency of suppression of the temperature-sensitivity. The frequency depends on the nature of the host, the structure of the plasmid and in some cases is enhanced by the presence of a cloned site-specific recombinase gene, binR. One explanation for suppression is the integration of the plasmid into the chromosome. The presence of Tn557 on the chromosome also greatly enhances the number of cadmium-resistant colonies.The chromosome of S. aureus strain PS80N has been shown by hybridization to carry at least most of Tn552 on a single EcoRl fragment. In several cases following selection for cadmium-resistance at the restrictive temperature, the plasmid pSl has been shown to interact with the EcoRl fragment at a particular plasmid site. The plasmid is presumed to have integrated into the chromosome and subsequent growth of bacteriophage followed by its use in transduction has revealed different types of recovered plasmids often with deletions commencing at a specific site. If the host chromosome carries Tn55/ then on transduction of the presumed integrated plasmid, a copy of Tn557 is found inserted into the transductant plasmid. For pSl the site of insertion of Tn557 is into the Tn552 transposase gene, whereas for pAD insertion occurs into different sites for different transductants.pSl and pΔ|D can form co-integrates with the resident cryptic plasmid pOX1054 of S. aureus 1054. The recombination occurs by a novel method. The data show that pSl or pΔD contribute the site for recombination and that the gene(s) for the protein(s) involved in recombination are encoded on pOX1054 or the 1054 chromosome. Integration of the plasmids into the chromosome of 1054 was not detected.