In order to study how visual categories are coded by the activities of single neurons, it is necessary to first demonstrate that the animal subjects can categorize the visual stimuli employed in the single-unit recordings. Thus, rhesus monkeys were trained in a visual categorization task designed to minimize rote learning of individual exemplars and to allow testing of transfer from old to novel exemplars of the category. The stimuli were presented during controlled fixation. The monkeys learned to distinguish complex colour images of trees from other objects and generalized from old to novel exemplars. An extensive series of tests with probe stimuli showed that simple form, colour and texture features had insufficient stimulus control to account for the categorization performance. Scrambling the images impaired categorization performance, suggesting that the configuration of stimulus components controlled the categorization. The animals also learned a fish/non-fish categorization, but more slowly than a tree/non-tree categorization. These results indicate that rhesus monkeys can learn to categorize socially neutral, complex, natural visual images and suggest that this categorization is based on a combination of low-level features.