A large-sample study (N = 717) is described in which two broad Big Five factor measures were compared with two narrow personality trait measures in the prediction of final grades in an undergraduate psychology course. The Big Five factors evaluated as predictors were Conscientiousness and Openness to Experience (or Intellect). The traits evaluated, which were constituents of the respective factors, were need for Achievement and need for Understanding. In each comparison, the lower level trait measure did better than its higher level factor measure in the prediction of course grades. We conclude that the aggregation of narrow trait measures into broad factor measures can be counterproductive from the points of view of both behavior prediction and behavior explanation.