This study reports on the feature of sport experience in relation to the process of athletes' ego identity formation viewed from Erikson's mutua1ity concept. The phases of mutuality was operationally defined as crisis, exploration and commitment. One hundred and sixty-four male college athletes in the physical education department who have been involved in competitive sports since pre-adolescence volunteered as subjects. Two questionnaires were given to these subjects. One of them was a scale of ego identity formation process that was intended to measure the level of three states in the process of the ego identity formation; crisis, exploration and commitment. Then, the level of three states were evaluated by the degree of subjects' interaction with thirteen areas in their life space (Lebenswelt); six sporting, six daily life and one area in common. The construction of this scale was based on the idea of Marcia's identity status aporoach and the operational definition of "mutuality". The other questionnaire was Endou's ego identity scale, used to estimate the degree of identity achievement. The principal findings were as follows: 1) The athletes have strong ego-involvement in sporting areas, and the area of athletic performance consistently receives especially high scores in all three states. 2) The level of exp1oration and commitment were significantly lower than that of crisis in the areas of "team-mates", "continuation in athletics", "coaches", and "sex". And the level of exploration was significantly lower than that of crisis and commitment in the areas of "choice of a job" and "life style and values". 3) The level of exploration and commitment appear to be the basis of the sense of ego identity. As for coping with the developmental task, the athlete is involved with almost the same areas as the adolescent in general, i.e., occupation, values, politics, and interpersonal relationship, which contributes to ego identity formation. 4) The areas of athletic performance, significance of their continuation in athletics, and relationship to coaches were found to be important areas for the athletes. However, in these areas development of "mutuality" (Erikson) which contributes to ego identity formation seems to be difficult.