Twenty-seven strains of gram-negative anaerobic bacteria isolated from patients with non-gonococcal urethritis (NGU-associated strains) were compared with reference strains of Bacteroides ureolyticus and representatives of other species of Bacteroides and Fusobacterium. Conventional biochemical tests, electrophoretic (PAGE) protein profiles and DNA base compositions of 29.5 +/- 1.2 mol% G + C indicated striking similarities between the NGU-associated strains and B. ureolyticus. Three of the reference strains previously assigned to that species were atypical in some respects as was one of the strains from urethritis and one strain, NCTC 10939, was incorrectly classified as B. ureolyticus. Casein hydrolysis by B. ureolyticus was the principal test difference between the two sets of strains. DNA-DNA hybridisation results of greater than or equal to 72% at optimum temperature conditions provided further evidence of the close relationship between B. ureolyticus and the NGU-associated strains and we conclude that these organisms should be identified as B. ureolyticus. There was insufficient evidence for recognising subspecies or biotypes within this revised concept of B. ureolyticus although several types (PAGE-type I, 17 strains; type II, 6 strains; type III, 3 strains) were detected which might provide the basis for future studies of hospital strains. The DNA base composition and hybridisation data indicated that B. ureolyticus was not related to other species of Bacteroides or Fusobacterium.