The occurrence of Neisseria gonorrhoeae, Chlamydia trachomatis and Mycoplasma genitalium was determined by molecular techniques in urine specimens from 182 black South African men who had symptoms and/or overt signs of urethritis. Eighty-six (47.3%) of these men were infected with N. gonorrhoeae. There were 185 men without overt evidence of urethritis, 16 (8.6%) of whom were also infected with N. gonorrhoeae. Of the 96 men who had non-gonococcal urethritis, 14 (14.6%) were infected with C. trachomatis, 16 (16.7%) with M. genitalium and only one with both microorganisms. In comparison, 15 (8.9%) of 169 men without overt urethritis and without N. gonorrhoeae were infected with C. trachomatis and 15 (8.9%) with M. genitalium, proportions that were about half the size of those in the group with overt urethritis.