Strains of a small, Gram-negative anaerobe described as 'NGU-associated' because of its apparent association with non-gonococcal urethritis (NGU), grew synergistically with anaerobic cocci on primary isolation. Subsequently, it was found that the addition of formate-fumarate to the media enhanced their growth. The organisms were asaccharolytic and, unlike most anaerobes, produced oxidase (cytochrome c) and tolerated 5% oxygen when grown in an atmosphere of 2% hydrogen, 7.5% carbon dioxide and 85.5% nitrogen. The strains examined appeared homogeneous culturally, morphologically and biochemically, and had some characteristics in common with Bacteroides ureolyticus (NCTC 10948) and Wolinella succinogenes (ATCC 29543). However, the latter was flagellated and had a DNA G + C content (46-49 mol%) that was different from B. ureolyticus (28 mol%) and the NGU-associated strains (28-30 mol%). These strains and B. ureolyticus shared many properties but electron microscopy of the NGU-associated strains revealed polar pili which were not seen on the strain of B. ureolyticus examined. Other tests which differentiated B. ureolyticus from the NGU-associated strains, being positive only for the latter, were lysine and ornithine decarboxylation, beta-haemolysis and growth in 5% oxygen: those that were negative for the NGU-associated strains were pitting of agar on primary culture and the production of acetate and lactate as metabolic end-products. Despite these differences, the NGU-associated strains may be biotypes or variants of B. ureolyticus, a species which we now consider needs further study and classification.