Dempster (Dempster; 1995; Dempster & Corkill, 1999) proposed that developmental changes in performance on Piagetian tasks could be related to changes in inhibitory efficiency more than to logical development. In this study, the negative priming paradigm was adapted to the class inclusion task in order to investigate the role of inhibition and knowledge levels in the development of class inclusion. Participants were pre-tested on two inclusion tasks, the standard Piagetian task and Markman's modification task, and assigned to different knowledge levels: empirical, and logical necessity. Children were then tested on a priming version of the class inclusion task. Results showed a negative priming effect, indicating that the irrelevant `subclass comparison strategy' it-as actively inhibited during the processing of the class inclusion task. This effect was found to vary as a function of knowledge levels, indicating that the need for inhibition was reduced when children had attained logical necessity.