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Sixty-three bacterial isolates from antarctic sandstone samples (Linnaeus Terrace, Asgard Range, McMurdo Dry Valleys) were screened for the presence of plasmid DNA. Twenty-seven percent of all the isolates (mainly Gram-positives) harbored one or more plasmids of low molecular mass (1.1-16.3 kb). Strain AA-341 contained plasmid pAA-1 (2.9 kb), as demonstrated by agarose gel electrophoresis and restriction endonuclease digests. This plasmid conferred resistance to chromium and ampicillin. It was not conjugative, but it could be transferred by electroporation to chromium- and ampicillin-sensitive strains AA-330, AA-338, and E. coli HB101. A restriction map of pAA-1 was constructed with HindIII, SalI, ScaI, AvaII, EcoRI, PvuII, BamHI, and DraI. Electrotransfer of this plasmid from E. coli HB101(pAA-1) to strain AA-330 was demonstrated. By natural genetic transformation, plasmid pAA-1 could be transferred into the sensitive strain AA-334, which thereby became resistant to chromium and ampicillin. The importance of such processes for the colonization of stressed environments is discussed.