Plasmid analysis of isolates from a small Paracoccus population revealed that all 15 representatives carried at least one endogenous plasmid of 23 or 15 kb in size, in addition to further plasmids of different sizes. It was shown by restriction analysis and hybridization that the 23 and 15 kb plasmids from the different isolates were identical or very similar to each other. By partial sequencing of pOL18/23, one of the 23 kb plasmids, a complete rrn operon with the structural genes for 16S, 23S and 5S rRNA, two genes for tRNA Ile and tRNA Ala within the spacer between the 16S and 23S rRNA genes, and a final tRNA fMet at the end of the operon were discovered. Expression of a green fluorescent protein gene (gfp) after insertion of a DNA fragment from the region upstream of the rRNA genes into a promoter-probe vector demonstrated that the rrn promoter region is functional. The rrn operon encoded by plasmid pOL18/23 is the first complete rrn operon sequenced from a strain of the genus Paracoccus, and only the second example of an rrn operon on a small plasmid.