Although there is a wealth of knowledge on categorization in infancy, there are still many unanswered questions about the nature of category representation in infancy. For example, it is yet unclear whether categories in infancy have well-defined boundaries or what knowledge about species categories young infants have before entering the lab. Using a morphing technique, we linearly altered the proportion of cat versus dog in images and observed how infants reacted to contrasts between pairs of images that either did or did not cross over the categorical boundary. This was done while equating between-category and within-category similarity. Results indicate that infants’ pre-existing categories of cats and dogs are discrete and mutually exclusive. Experiment 2 found that inversion caused a disruption in processing by 6.5- but not 3.5- month-old infants, indicating a developmental change in category representation. These findings demonstrate a propensity to dichotomize early in life that could have implications for social categorizations, such as race and gender. Furthermore, this work extends previous knowledge of infant categorical perception by demonstrating a priori knowledge of familiar species categories and the boundaries between them.