Total DNA of virulent and avirulent strains of Rhodococcus fascians was resolved by pulsed-field gel electrophoresis (PFGE) into a discrete number of fragments by digestion with the endonucleases AseI and DraI. Restriction endonucleases PacI, PmeI, and SwaI yielded no fragments upon digestion of R. fascians genome, and all the other tested endonucleases recognizing 6 bp released too many fragments. The genome size was 5.6 megabases for the type strain R. fascians DSM 20669, and 5.8 megabases for the virulent R. fascians D188 strain. However the genome size of R. fascians CECT 3001 (NRRL B 15096) was 8.0 megabases. No linear chromosome in the megabase range was observed under pulse conditions in which Saccharomyces cerevisiae and Schizosaccharomyces pombe chromosomes were perfectly resolved, suggesting that the R. fascians chromosome is circular A new linear plasmid pIRN640 of 640 kb was found in the avirulent R. fascians CECT 3001 that did not hybridize with a probe internal to the fas region of pFiD188 known to be involved in plant pathogenicity in the virulent strain R. fascians D188. Virulence was correlated in all strains tested with the presence of the fas region, The AseI and DraI bands corresponding to the extrachromosomal elements were identified providing the basis for a physical map of this organism.