The Impostor Phenomenon (IP) is an experience of phoniness found in high achievers. This study investigates the personality and motivational characteristics that may predispose one to private feelings of unworthiness when one is, by all objective criteria, very successful. The relationship of the IP to achievement behavior was the specific focus of this study. The Personality Research Form, the Myers-Briggs Type Indicator, and Clance's IP Scale (1985) were administered to 153 experimental subjects. The findings indicate that the IP correlates best with the PRF traits of Social Recognition Succourance, Aggression, and Defendence. The traits least likely to be correlated with the IP were Affiliation, Autonomy, Change, Play, Desirability, and Endurance. The IP was also positively correlated with Introverted Personality types who use sensing as their primary auxiliary function. It appears that Impostors have a high need for social validation and may be using a performance goal motivational pattern, which is maladaptive and prevents them from internalizing success. Implications of this study and suggestions for further research are offered.