Similarity is a core concept in theories of object recognition, categorization, and reasoning. It is often conceptualized as a geometric distance in a multidimensional stimulus space. However, research in humans has revealed that similarity judgments involve more than a simple distance calculation and tend to be asymmetric when stimuli differ in factors such as prototypicality. For example, most people judge 99 to be more similar to 100 than 100 to 99. Up to now, it was not known whether such asymmetries might also occur in nonhuman subjects. This study reveals asymmetries in the pattern of errors made by four rhesus monkeys in a temporal same/different task. Monkeys usually perceived a smaller difference between two different stimuli when the first stimulus in a trial was less prototypical than the second, just as what was found previously for human subjects. The pattern of asymmetries differed between monkeys, and a control study showed that such variability is also present for human subjects. We propose that known neurophysiological mechanisms can account for asymmetry in the stimulus comparisons of both species. Thus, seemingly complex phenomena that occur when human subjects rate stimulus similarity are also present in macaques' similarity judgments and could be based on relatively simple mechanisms.