Two staphylococcal plasmids, pWG4 and pWG25, encode production of a diffusible pigment and resistance to erythromycin and spectinomycin. The former was found occurring naturally in a clinical isolate of Staphylococcus aureus and the latter in S. epidermidis. Both plasmids are conjugative, capable of high-frequency, interspecies transfer, only isolated in the open-circular form and identical in molecular weight and pattern of restriction-endonuclease fragments. The only difference between the plasmids is in the expression of resistance, pWG4 encoding inducible and pWG25 constitutive erythromycin resistance. The resistance determinants of both plasmids behave as hitch-hiking transposons in cultural conditions that favour phage-mediated or phage-independent conjugation, always inserting a copy of themselves into the recipient's chromosome, except in S. epidermidis in which the chromosomal insertion site may be absent. The resistance determinants have been cloned and located on a 4 X 7 kbp EcoR1/HindIII restriction fragment which has a restriction map similar to that of the right arm of Tn554 (Murphy and Lofdahl, 1984). The hitch-hiking transposon of plasmid pWG25 has been designated Tn3853.