The primary aim of this study was to determine whether anaerobes played a significant role in the aetiology of non-gonococcal urethritis (NGU). Chlamydia trachomatis and Ureaplasma urealyticum are responsible for about 50-60% of cases of NGU, so it is plausible that anaerobes may play a role in those cases with no known aetiology. The occurrence of the anaerobe Bacteroides ureolyticus in the genito-urinary tract of men with and without disease was considered, and microbiological and clinical studies performed in conjunction with the recognised aetiological agents, demonstrated a striking association between this anaerobe and NGU. The primary cultivation of B.ureolyticus on non-enriched blood agar was practicable despite its fastidious metabolism, in particular, a formate-fumarate energy-requirement for growth. Several improved media were developed: namely, a transport medium of low redox-potential, liquid and enrichment media, solid media and a selective agar medium. Results of the various media are presented. The transport medium was effective in maintaining the viability of anaerobes during transportation, and together with the sampling procedure, produced optimal recovery from swabs. The selective medium containing vancomycin, polymyxin and trimethoprim was equally effective in facilitating the recovery of B. ureolyticus from clinical specimens. Methods for the identification of Gram-negative anaerobes in general are described. They were based on biochemical tests, antibiotic-disc resistance tests, and on tests of tolerance to bile salts and dyes. Such a scheme enabled the identification of unknown anaerobes. However, the definitive identification of several strains of B.ureolyticus posed a few problems when using a large number of diagnostic tests, mainly because of the heterogeneity of the species. Methods of classification by numerical taxonomy, and characterisation aids such as ultrastructure and the electrophoretic pattern of major cell-proteins are described. Serological studies showed that at least 30% of chlamydia-negative cases of NGU were probably due to B. ureolyticus, and organ culture studies demonstrated toxin-mediated damage to mucosal epithelia. The significance of B. ureolyticus as a potential pathogen and probable aetiological agent in NGU is discussed.