The 30 sixth and 30 ninth grade students, all of whom attended one rural school district in central Illinois, not only displayed a wider variety of behaviors than reported in the Inhelder and Piaget study (1958) investigating the development of logical thinking in children, but the behavioral components served to fill significant gaps in the description reported in the original study. The main Piagetian descriptions of the stages of cognitive development under investigation were supported by the findings of the study. The focus was on (1) developmental problems relating to the transition between the concrete and formal operational stages, and (2) methodological problems in operationalizing Inhelder and Piaget's stage classification procedures. The inter-rater reliability coefficient, obtained from a comparison of the independent stage classifications of the two judges was .96 and .98 for the ratings on the chemicals and balance tasks (the two Piagetian tasks administered to each subject) respectively. It was concluded that the classification system could be used objectively by an independent judge. The extensive training period required for the rater did imply that further simplification of the procedures would be useful. (EB)