Summary.A collection of 50 clinical isolates of Bacteroides was examined for plasmid deoxyribonucleic acid content. An attempt was then made to correlate the presence of plasmids with a specific phenotypic property. Of the 20 Bacteroides which contained plasnlids, 18 were found to harbour plasmids of 98 megadaltons. The most common plasmid had a molecular weight of 4 8 megadaltons and was found in 9 strains. Most strains had multiple plasmid bands. All strains were examined for resistance to penicillin, cefoxitin, erythromycin, tetracycline, sulphamethoxazole, clindamycin, chloramphenicol, arsenate, silver, cadmium, mercury, chromium, lead, nickel and cobalt, and for the production of /3-lactamase, heparinase, 'deoxyribonuclease, haemolysins and bacteriocins. Using a Chi-squared analysis, there was no statistically significant correlation between any of these phenotypic traits and the presence of plasmids, except bacteriocin production. A total of 15 out of 20 (75%) of plasmid-containing strains produced bacteriocins while only 10 out of 30 (33%) of plasmid-free strains were capable of bacteriocin production (x^, p < 0 005). Attempts to transfer or cure resistance to antibiotics and heavy metals or bacteriocin production were not successful.