Establishing what information is actively maintained in working memory (WM) and how it is represented and controlled is essential to understanding how such information guides future behavior. WM has traditionally been investigated in terms of the maintenance of stimulus-specific information, such as locations or words. More recently, investigators have emphasized the importance of rules that establish relationships between those stimuli and the pending response. The current study used a mental arithmetic task with fMRI to test whether updating of numbers (i.e., stimuli) and updating of mathematical operations (i.e., rules) in WM relies on the same neural system. Results indicate that, while a common network is activated by both types of updating, rule updating preferentially activates prefrontal cortex while number updating preferentially activates parietal cortex. The results suggest that both numbers and rules are maintained in WM but that they are different types of information that are controlled independently.