Unlike Corynebacterium diphtheriae and Propionibacterium acnes, the pleomorphic gram-positive rods known as diphtheroids are generally regarded as nonpathogenic contaminants of the human external eye. We reviewed five years of microbiology records at Wills Eye Hospital and studied a series of eight cases of apparently infectious keratitis associated with heavy growth of diphtheroids on cultures of ulcer scrapings. All of these cases included indolent ulcers that occurred almost exclusively in elderly patients (mean age, 72 years; range, 11 to 92 years). All patients had preexisting ocular conditions that compromised the corneal surface such as exposed corneal sutures, eyelid surgery, aphakic extended wear contact lenses, viral keratitis, and diabetes mellitus. No other pathogens were isolated. All infections responded well to antibiotic therapy with all organisms sensitive to cefazolin and all but one sensitive to gentamicin.