This study was designed to examine systematically the relationship of Erikson's concept of ego identity to psychosocial effectiveness as actually demonstrated in daily living. Two groups of Navy recruits, one composed of individuals making a highly adequate psychosocial adjustment and the second containing persons demonstrating minimally adequate adjustment, served as Ss. Highly significant differences were obtained on an operational measure of ego identity. Evidence of construct validity was adduced in terms of the predicted relationship of ego identity and a self-concept measure as well as through inter-correlation of subscores on the ego identity instrument. The results lend support both to the value of Erikson's theory in the systematic study of personality and his position that an adequate ego identity is necessary for a person to cope effectively with his social and cultural environment.