A virulent, hemolytic strain of Moraxella bovis was instilled into the conjunctival sac of gnotobiotic calves, and corneas and conjunctivae were sampled serially after infection. Histologically, M. bovis was first seen within swollen corneal and conjunctival epithelial cells; conjunctival lesions were prominent near lid margins although bacteria were occasionally seen within superficial epithelium in other areas. Corneal ulcers increased in severity with time and were characterized by fibrin deposits, neutrophils, and bacteria in the stroma. Examination of corneas by scanning electron microscopy showed M. bovis in pits on the surfaces of dark epithelial cells, enmeshed in degenerate epithelial cells and within erosions and a unilateral ulcer. Unilateral and bilateral ulcers were seen in later stages, but bacteria were rare. Examination of palpebral conjunctivae showed M. bovis in pits on the surfaces of dark and lighter epithelial cells and within erosions near the lid margins. Ultrastructurally, M. bovis was seen within swollen corneal and conjunctival epithelial cells; many epithelial cells had undergone cytolysis. In corneal stroma, M. bovis was frequently seen among collagen fibrils, within neutrophil phagosomes, and associated with cellular debris. This study demonstrated that a virulent strain of Moraxella bovis is cytotoxic for and can invade corneal and conjunctival epithelial cells, replicate, and can cause keratoconjunctivitis in calves in the absence of predisposing environmental factors.