The Graphical User Interface (GUI) is one of the most revolutionary changes to occur in the evolution of modern computing systems. ... This revolution has increased the accessibility and usability of computersystems to the general public ..." (Mandelkern, 1993, p. 37). In the excitement over Graphical User Interfaces, developers often overlook the fact that they are making assumptions about how users best process information. A key assumption in the GUI is that users are effective processors of spatial information. In fact, there are individual differences in how well people process information spatially. This paper describes an experiment that measures these differences and looks for their effects on users' abilities to learn command line and GUI interfaces for simple file management tasks.