A cluster analytic approach was used to determine possible age differences in the structure of personality as measured by the Cattell Sixteen Personality Factor Questionnaire. Subjects were 969 adult male volunteers, originally divided into three groups: 25 to 34, 35 to 54, and 55 to 82. Two familiar dimensions of personality, Anxiety-Adjustment and Introversion-Extraversion, were found to be cross sectionally stable. A third cluster dimension, interpreted as openness to experience, showed age-related differences. Young subjects showed openness to feelings; middle-aged subjects showed openness to ideas; and old subjects showed a balanced openness to both feelings and ideas. Repartitioning of the sample into two groups (ages 25 to 43 and 44 to 82), and three groups (25 to 39, 40 to 47, and 48 to 82) confirmed these age differences in the third cluster. The interpretation of the third cluster is discussed in terms of Jung's concepts of psychological functions.