The microbial aetiology of genital ulcers was assessed in 100 black men attending a sexually transmitted disease (STD) clinic in Durban, South Africa. Forty patients harboured Haemophilus ducreyi, one hepes simplex virus, and one Neisseria gonorrhoeae. Syphilis was diagnosed in 44 patients on the basis of dark field microscopy or positive syphilis serology test results, or both. Of these 44 patients, eight also harboured N ducreyi, one herpes simplex virus. Lymphogranuloma venereum was diagnosed in one patient. No cause of ulceration could be found in the remaining 16 patients.