Weakly beta-haemolytic human intestinal spirochaetes (w beta HIS) isolated in Italy showed an extrachromosomal band migrating at 4.3 kb after agarose gel electrophoresis of total DNA. The band was observed within 4 strains (HRM2, HRM4, HRM7 and HRM14) among the 7 w beta HIS analysed and was resistant to proteinase K and RNase treatment, whereas after purification it was completely digested by incubation with DNaseI. The origin, the structure and the significance of this extrachromosomal DNA are at present unknown. In addition to w beta HIS, the reference strain P18A of Serpulina hyodysenteriae was comparatively analysed and a 6.5 Kb extrachromosomal DNA element was found, as expected. This finding proves that the experimental conditions we used while searching for extrachromosomal DNA within w beta HIS were appropriate. To the best of our knowledge, this is the first report on the detection of extrachromosomal DNA from w beta HIS.