Metacognition research has focused on the degree to which nonhuman primates share humans' capacity to monitor their cognitive processes. Convincing evidence now exists that monkeys can engage in metacognitive monitoring. By contrast, few studies have explored metacognitive control in monkeys, and the available evidence of metacognitive control supports multiple explanations. The current study addresses this situation by exploring the capacity of human participants and rhesus monkeys ("Macaca mulatta") to adjust their study behavior in a perceptual categorization task. The study found that humans and monkeys increased their study for high-difficulty categories, suggesting that both share the capacity to exert metacognitive control. (Contains 3 tables and 2 figures.)