One's self-definition is hypothesized to change, in the service of self-esteem maintenance, as a function of the relative performance and the psychological similarity (closeness) and dissimilarity (distance) of others. Specifically, a dimension should become less self-definitional if another performs better on that dimension, particularly if that other is psychologically similar. Fifty-two subjects were given an opportunity to perform on a social sensitivity task and an esthetic judgment task with an experimental confederate. Half of the subjects were led to believe (on the basis of pretest materials) that the confederate was very similar to them, i.e., close, and the remaining subjects were led to believe that the other was dissimilar, i.e., distant. Subjects found that they performed at the same level as the confederate on one of the tasks and that the confederate outperformed them on the other task. The dependent variable, self-definition, was measured by task choice, an interview regarding the self-definitional character of the two dimensions, and the subjects' self-descriptions on these dimensions. The hypothesis is supported on the choice and the self-description measures.