Opportunity has been afforded us for the study of influenza prevalence in three different population groups in the city of Chicago: (1) The Student Army Training Corps at the University of Chicago; (2) The High and Elementary Schools of the University of Chicago, and (3) the Chicago Telephone Company. These several groups are quite distinct as regards age, degree and nature of association of the iadividuals within each group, and general opportunities for exposure to infection. So far as known there was no point of contact between the groups.