Evidence is summarized here for the degree to which the field of communication study is divided into two Subdisciplines: mass communication versus interpersonal communication. This division is expressed (a) in the general lack of cross-citations between five mass communication journals and five interpersonal communication journals, (b) by separation of the two Subdisciplines in communication associations, and (c) by the awarding of doctoral degrees in programs specializing mainly in interpersonal communication or in mass communication. The historical and other reasons for this bifurcation of communication study and the functions and dysfunctions of this division are discussed.