Individuals with autism have difficulty integrating information and generalizing previously learned concepts to new situations. It was hypothesized that these problems result from an underlying impairment in category formation. Persons with autism may not abstract a summary representation (a prototype) during category learning and, instead, may form categories by memorizing a list of rules. Children with autism, Down syndrome, and normal development participated in one set of category learning tasks that could be solved using a rule-based approach and a second set of tasks in which there was no rule that defined category membership (prototype tasks). In the rule-based tasks, all groups were successful at using a rule to learn a new category. In the prototype tasks, only the typically developing children were able to learn a new category. Neither the persons with autism nor the persons with Down syndrome appeared to develop a prototype during category learning. These data suggest that persons with autism and Down syndrome have difficulty categorizing new information by forming prototypes and, instead, tend to rely on a rule-based approach to learning.