Several theories predict that supervisors' expectations about subordinates' performance may bias their subsequent evaluations of those subordinates. Data from 49 supervisor-subordinate dyads were collected within a longitudinal design. Results suggest that prior expectations interact with actual performance to affect ratings, implying that supervisors respond negatively to disconfirmation of expectations. No support for the notion that an attributional mechanism can explain this reaction to disconfirmation emerged. These findings suggest that certain affective components of appraisal may be important for understanding bias.