In a "class inclusion" task, a child must respond to stimuli as being involved in two different though hierarchically related categories. This study used a Relational Frame Theory (RFT) paradigm to assess and train this ability in three typically developing preschoolers and three individuals with autism spectrum disorder, all of whom had failed class inclusion tests. For all subjects, relational training successfully established the target repertoire and subsequent testing demonstrated both maintenance and generalization. Limitations and future research directions are discussed.