Two experiments examined whether text composition engages verbal, visual, and spatial working memory to different degrees. In Experiment 1, undergraduate students composed by longhand a persuasive text while performing a verbal, visual, or spatial concurrent task that was presented visually. In Experiment 2, participants performed a verbal or spatial concurrent task that was aurally presented. Writing performance was not disrupted differentially across the three tasks. Performance on all concurrent tasks showed fewer correct responses and longer RTs relative to single-task, baseline data. However, the demands on visual working memory were as high as those on verbal working memory, whereas demands on spatial working memory were minimal. The findings help to delineate the roles of the verbal, visual, and spatial working memory in written composition.