The study of multiple personality is important because this disorder is severely disruptive to the individual and because further insights into other behavior disorders and the development of normal personality and behavioral functioning may emerge. A 26-year-old female experienced a conscious personality which determined the extent to which both she and a second personality controlled behavior. Her second personality was conscious only in the controlled behavior situation; the first personality appeared during the other time periods as blackouts. The dominant personality emerged following a traumatic childhood event and gradually assumed control of behavior for approximately 80% of a given time period. As in virtually all reported cases of multiple personality, striking and complementary differences in the sociability, roles, interests, temperaments, and levels of functioning were exhibited in the client's personalities. Multiple personality individuals often possess ambivalent attitudes toward each other and discplay additional psychopathology. Their childhoods are usually characterized by severe family discord or instability and by obvious psychopathology in at least one parent. Some success in treating multiple personalities has been reported by therapists with psychoanalysis, behavioral therapy, hypnotherapy, family systems therapy, and general supportive therapy approaches. (NRB)