Hemophilus ducreyi was isolated from the ulcer of one of 33 patients in whom chancroid had been diagnosed clinically. Herpes simplex virus was isolated from 16 of these patients. A presumptive diagnosis of chancroid was made in one of three sexual partners of the only index patient with culture-proved chancroid. No chancroidal lesions were found in any of 39 sexual partners of the other 32 index patients. Genital cultures for herpes simplex virus were positive from four of 14 sexual partners of index patients with genital herpes simplex infection. The differential diagnosis of chancroidal genital ulcers is facilitated by herpes simplex virus cultures and a new selective H ducreyi culture medium.