Abstract Recent research has indicated that infants are capable of responding to stimuli in a manner indicating that they categorize them. Infant perception of orientation was examined within a framework of categorization. In one experiment, it was shown that 4-month-old infants generalized habituation from one range of oblique grating stimuli to another, consistent with the interpretation that any two oblique stimuli were perceived as more similar than a vertical and an oblique. Fourmonth-old infants' generalization was not due to a simple inability to discriminate between obliques (Experiment 2) so the results of Experiment 1 reflect in large part true categorization behavior and not categorical perception. Results for 2- and 3-month old infants suggest that “vertical” serves as a reference stimulus in infant orientation perception such that gross distinctions between vertical and nonvertical precede the development of the “oblique” category. The category boundary between oblique and vertical did not successfully predict better between-than within-category discrimination in 4-month-old infants (Experiment 3) under the conditions of these experiments.