An examination of the distribution of plasmids carried by copper-tolerant bacteria from a freshwater stream revealed that >60% carried at least one plasmid, and that large plasmids (>100 kb) were predominant. A total of 10 copper-tolerant bacteria carrying the 54-kb plasmid, pFBA20, were detected at four sampling sites within the stream and, on consecutive occasions, at one site throughout a 1-year sampling period. The detection of this plasmid provides evidence that related plasmids can, under no apparent selective pressure, survive and disperse within the bacterial community. Two of the isolates that carried pFBA20 were phenotypically distinguishable. This would suggest that pFBA20 is transmissible.