We investigate interference between declarative and procedural representations in working memory (WM). Declarative representations are objects of thought, whereas procedural representations provide the (cognitive) actions to work upon these objects. In eight dual-task experiments we varied the number of representations to be maintained in WM (memory load). In Experiments 1–4, we varied declarative and procedural load separately in the two tasks used. In Experiments 5–8, only declarative or procedural load was manipulated in both tasks employed. We measured how much performance in the currently relevant task was impaired by increasing the load in the currently irrelevant task. These cross-task load effects were larger for Experiment 5–8 compared to Experiment 1–4. Yet, in task-switch trials we also obtained cross-task load effects in Experiment 1–4. Our findings support the distinction of declarative and procedural WM as largely independent sub-systems or distinct representational spaces.