The article presents the idea that individuals possess three main mental representations of distributions regarding group categories: trait differentiation, person differentiation, and the interaction between trait and person variabilities. These representations are called Implicit Distribution Theory. The study examined the stability of these distributional notions, their interrelationships, and their relation to subsequent judgments. The implicit distribution theories of 89 students regarding university professors were captured twice by asking them to distribute 100 imaginary professors on 5-point scales for each of 13 attributes commonly used in professors' evaluations. Several weeks afterwards 50 of these subjects rated four known professors on the same attributes. Test-retest reliability was quite high for trait and person differentiation, and these constructs were independent. Person differentiation significantly predicted parallel features in raters' judgments of actual performance, while trait differentiation yielded significant correlations with most distribution measures of actual ratings. The findings are related to the concepts of mental representation, cognitive complexity, and cognitive biases in person perception as well as to their potential applications in organizational settings.