This study explored how transitioning from a single-sex to a coeducational high school affected the self-concept of students in a private, Catholic high school. The initial data gathered were from a single-sex, all female population. Thereafter, data reflect an influx of males as the school transitioned from a single-sex to a coeducational high school. Subjects were volunteer students in grades 9 through 12 who completed the Self Description Questionnaire each year from 1989 through 1992. The results indicated that males maintained significantly higher total self-concepts than did females while no significant differences were noted in total self-concept as students moved from a single-sex to a coeducational school. In terms of general self-concept, males again demonstrated significantly higher self-concepts than did females, while general self-concept improved significantly as all students progressed from freshmen to seniors. Positive self-concept in the mathematics area was significantly in favor of males in comparison to females, while underclass students demonstrated significantly higher mathematics self-concepts than did their senior counterparts. Males also scored significantly higher than females in self-concept of physical abilities. (NB)