Adherence of Pseudomonas aeruginosa to the cornea is the first step in the pathogenesis of Pseudomonas corneal ulceration. To determine the roles of epithelial and stromal adherence, a quantitative in vivo model of adherence was developed in the mouse. More than twice as many organisms were recovered from mouse corneas in which two linear abrasions were created compared with corneas totally denuded of epithelium. Scanning electron microscopy demonstrated poor adherence of Pseudomonas to bare stroma, but large numbers of organisms were seen adhering to the injured edge of epithelium. This study supports the concept that corneal trauma predisposes to Pseudomonas corneal ulceration by creating an injured epithelial edge as a site for adherence of the organisms, rather than by exposing areas of bare stroma susceptible to penetration by the organisms.