This study demonstrated reinforcement control over pronounced feminine behaviors in a male child. The clinical history of S paralleled the retrospective reports of adult transsexuals, including (a) cross-gender clothing preferences, (b) actual or imaginal use of cosmetic articles, (c) feminine behavior mannerisms, (d) aversion to masculine activities, coupled with preference for girl playmates and feminine activities, (e) preference for female role, (f) feminine voice inflection and predominantly feminine content in speech, and (g) verbal statements about the desire or preference to be a girl. S was treated sequentially in the clinic and home environments by his mother who was taught how to reinforce masculine behaviors, and to extinguish feminine behaviors. During this treatment, S's feminine behaviors sharply decreased and masculine behavior increased. A multiple-baseline intrasubject design was used to insure both replication and reliable identification of relevant treatment variables. (Author/LAA)