This report describes a large outbreak of conjunctivitis caused by an unencapsulated strain of S. pneumoniae. During the outbreak, 698 students received diagnoses of conjunctivitis, including 22 percent of first-year students. More than 1 in 20 students carried the outbreak strain at the time of our investigation. Students carried the outbreak strain after conjunctivitis symptoms had resolved; some students carried the strain without having symptoms. Analysis of risk factors suggested that the outbreak strain was highly transmissible from person to person; students who reported having had close contact with a student with conjunctivitis and those who participated in activities that involved physical contact were more likely to become infected. The organism was probably spread through respiratory secretions from students carrying the bacteria, as is the case with typical pneumococci, and through contact with eye secretions of students with conjunctivitis.rather than other pneumococcal syndromes. The high attack rate suggests that few students had preexisting immunity to this strain. The absence of a capsule may limit this strain's ability to cause more serious infections.