In this study, three experiments examined the impact of stimulus similarity on the benefits of spacing and interleaving for long-term memory. Two laboratory-based experiments (Experiments 1 and 2) and one classroom-based experiment (Experiment 3) were conducted. In Experiment 1, an advantage for interleaving relative to massing stimuli during encoding was observed as a greater proportion of correct responses on a categorization test for birds and paintings. This advantage was significantly greater when the stimuli were similar (e.g., interleaving different bird categories) rather than dissimilar (e.g., interleaving bird and painting categories). In Experiment 2, no advantage of interleaving relative to massing stimuli was observed in either the proportion of correct responses or response times on a categorization test for abstract visual stimuli. In Experiment 3 no significant differences between massed and interleaved study conditions were observed on a categorization test for textual materials. Although the results from this study are preliminary, the pattern of results in Experiment 1 suggests that interleaving may be most beneficial when the interleaved stimuli are similar rather than dissimilar.