Individual differences among users of a hierarchical file system were investigated. The results of a first experiment indicated that psychometric tests of vocabulary and spatial visualization were the best predictors of task performance, accounting for 45% of the variance in the data. The spatial predictor was found to be the most influential. This was dramatically illustrated by the fact that, on the average, subjects with low spatial ability took twice as long to perform the task as those with high spatial ability. Surprisingly, experience alone did not predict task performance. A comparison of the frequency of command usage between subjects with high and low spatial abilities revealed that those with low spatial ability were getting lost in the hierarchical file structure. Based on the concept of visual momentum, two changes to the interface were proposed. The changes consisted of a partial map of the hierarchy and an analog indicator of current file position. A second experiment compared the performance of users with high and low spatial abilities on the old Verbal interface and the new Graphical interface. The Graphical interface resulted in changes in command usage that were consistent with the predictions of the visual momentum analysis. Although these changes in strategy resulted in a performance advantage for the Graphical interface, the relative performance difference between High and Low Spatial groups remained constant across interfaces. However, the new interface did result in a decrease in the within-group variability in performance. / Master of Science