This study examined the mechanisms and issues that are important for personality development in athletes, and the effect of sports experience on personality development. We hypothesized that experiences of crisis, exploration, and commitment in athletic and daily life events promote personality development. Junior high school athletes (206 boys, 196 girls) and senior high school athletes (217 boys, 163 girls) completed a questionnaire assessing their experience of crisis, exploration, and commitment for four issues in the athletic domain, and for six issues in the daily life domain. Levels of ego development, as an index of individual personality development, were assessed with the Sentence Completion Test. Multiple regression analyses with crisis, exploration, and commitment as predictor variables and ego development as a criterion variable revealed that, for male junior high school athletes, there were significant relationships between predictor and criterion variables with regard to teammates, coaches, athletic performance, studying, and life style/values; for male senior high school athletes, there were significant relationships between predictor and criterion variables with regard to teammates, coaches, athletic performance, and future occupation/life course; for female junior high school athletes, there were significant relationships between predictor and criterion variables with regard to teammates, life style/values, and friends of the same sex; for female senior high school athletes, there were significant relationships between predictor and criterion variables with regard to teammates, coaches, future occupation/life course, life style/values, and friends of the same sex. For all groups, significant relationships between predictor and criterion variables were exhibited for at least one issue in both the athletic domain and the daily life domain. These results provide support for the hypothesized mechanisms of personality development in athletes, and suggest that sports experiences during the junior and senior high school years affect personality development in both boys and girls.