Electron microscopy was used to examine Haemophilus ducreyi adherence to and entry into eukaryotic cells of genital origin. A clinical H. ducreyi isolate (90-244) adhered in snake-like whorls to the surfaces of cervical carcinoma cells (HeLa 229), endometrial adenocarcinoma cells (HEC-1-B), and human neonatal foreskin fibroblast (HFF) cells. A prototype strain of H. ducreyi (CIP542) adhered in randomly organized clumps on the surfaces of HFF. Strain 90-244 entered HFF and HEC-1-B cells but did not enter HeLa cells. The H. ducreyi in the HFF cells at 2 h were partly surrounded by a membrane consistent with that of a phagocytic vacuole. At 2 h, strain CIP542 was found in interstitial spaces between the HFF cells and also in the cytoplasm of the cells. After 7 and 24 h, both strains of H. ducreyi were found in the large interstitial spaces between the HFF cells, in the cytoplasm, and extracellularly. This model of in vitro H. ducreyi infection of eukaryotic cells will allow for more specific study of factors that determine the virulence of H. ducreyi.