Hybridization was used to investigate the distribution of enterococcal plasmid sequences among 306 strains of Enterococcus and Streptococcus spp. isolated from faeces of humans of various ages. As DNA probes for the survey three plasmids, whose DNAs did not hybridize each other and designated as pMS13, pTW34 and pHK30, were selected from plasmids borne in Ent. faecalis. pTW34 DNA hybridized only with DNAs from enterococci, with high frequency in Ent. faecalis and low frequency in Ent. faecium. pMS13 DNA hybridized with DNAs of all Enterococcus spp. tested and with Strep. bovis, Strep. equinus and Strep. salivarius. Eighty-five percent of Ent. faecium isolates had sequences homologous to pMS13 but in the other species the values were less than 60%. Some enterococci had DNAs which hybridized with the pHK30 probe. The different distribution of the three DNA sequences indicates the possibility that plasmid DNAs encode advantageous phenotypes for the colonization of bacteria in the lumen of the bowel.