The interaction between Pseudomonas aeruginosa and the rabbit corneal epithelium during the first hour after inoculation was studied using scanning and transmission electron microscopy. Fifteen minutes after inoculation, adherent bacteria were found on damaged or exposed basal epithelial cells at the edge of an epithelial defect, and this adherence was the result of an interaction between the bacterial and epithelial cell membranes. Thirty minutes after inoculation, the adherent bacteria began to penetrate the epithelial cells by the formation of "pockets" surrounding the organisms, and after an additional 15 minutes the bacteria further penetrated the cells as the pockets began to fill in with cellular material. One hour after inoculation, only rare bacteria were seen on the cell surfaces or within the margins of the epithelial defect. Transmission electron microscopy, however, revealed many intracellular bacteria or bacteria that had migrated between the basal epithelium and corneal stroma. Therefore, it appears that the initiating events in Pseudomonas corneal ulceration involve adherence to the damaged or exposed basal epithelial cells by an interaction between the bacterial and cellular membranes, after which the organisms are engulfed by the epithelial cell and reach the corneal stroma by a process of transcellular migration.