During a field investigation of acute conjunctivitis in the lower Rio Grande Valley of Texas bacteriological cultures were made from more than 100 cases. Bacteria of the genus Hemophilus, apparently the main cause of the infection, were isolated from the conjunctiva of approximately one-half of the cases. Some strains were readily identified as Hemophilus influenzae. The majority, although resembling H. influenzae in some respects, were found to be different from it and to be like the so-called Koch-Weeks bacillus described by early investigators (Koch, 1883; Weeks, 1886, 1887; and others). The relation of the Koch-Weeks bacillus and H. influenzae has been the subject of much controversy and the two have been grouped as one species by many workers. In this paper the characteristics of the Koch-Weeks bacillus and ways of separating it from H. influenzae will be presented.