In mental rotation, a mental representation of an object must be rotated while the actual object remains visible. Where is this representation stored while it is being rotated? To answer this question, observers were asked to perform a mental rotation task during the delay interval of a visual working memory task. When the working memory task required the storage of object features, substantial bidirectional interference was observed between the memory and rotation tasks, and the interference increased with the degree of rotation. However, rotation-dependent interference was not observed when a spatial working memory task was used instead of an object working memory task. Thus, the object working memory subsystem--not the spatial working memory subsystem--provides the buffer in which object representations are stored while they undergo mental rotation. More broadly, the nature of the information being stored--not the nature of the operations performed on this information--may determine which subsystem stores the information.