During a 30-year period, causative organisms were identified on Gram's stain and culture in 547 of 677 cases of infectious corneal ulcers. Even if no organisms were seen on Gram's stain, culture results were often positive. Staphylococcus was the most common isolate; Moraxella, Pseudomonas, and Streptococcus pneumoniae were the next most frequent pathogens. Only 1% of cases were fungal. Of some help in identifying the causative organism were locations, presence or absence of hypopyon, and perforation of the ulcer. Most marginal lesions were a result of staphylococcal infection; central lesions were more likely to be Gram-negative ulcers. Fungal ulcers were identified by laboratory workup and not by clinical characteristics. Moraxella infection was observed almost exclusively in malnourished patients. Pseudomnas ulcers were often seen in patients with large body burns or patients receiving respiratory assistance. Gentamicin and neomycin were the most effective treatments.