The categorization performance of monkeys on a unidimensional perceptual categorization task was examined with reference to decision bound and exemplar theories of categorization. Three rhesus monkeys were presented with stimuli varying along a single dimension, the displacement of a target light from a fixation point. Left or right saccade responses were probabilistically reinforced according to one of three functions, two of which were nonmonotonic at one end of the stimulus space. The monkeys all showed a monotonic increase in response probability as a function of target light displacement in this region, consistent with decision bound theory. Fits of a single-boundary model (GRT; Ashby & Gott, 1988) and two exemplar models—one using a probabilistic response function (GCM; Nosofsky, 1986), the other using a deterministic response function (DEM; Ashby & Maddox, 1993)—revealed overall support for the decision bound model. The results suggest that monkeys used a perceptual decision boundary to perform the task.