The role of stimulus similarity as an organizing principle in immediate memory was explored in a series of experiments. Each experiment involved the presentation of a short sequence of items. The items were drawn from two distinct physical categories and arranged such that the category changed after each pair of items. Following list presentation, one item was re-presented, and the subjects tried to recall the item that had directly followed it in the list. Recall was more probable if the re-presented item and the item to be recalled had been presented in the same sensory modality (i.e., auditory or visual), the same voice, or in the same spatial location than if they had been presented in a different modality, voice, or location. It is concluded that stimulus similarity plays a broader role in organizing immediate memory than is generally assumed.