Performance of 195 seventh-, eighth-, and ninth-grade students on the Test of Logical Thinking (TOLT) was used to identify differences related to five reasoning modes among the three classes and between male and female students. TOLT scores revealed substantial deficiencies in the development of student reasoning abilities, and only ninth-grade students had significantly better (p < .05) performance than seventh-grade students which was related to proportional reasoning problems. There were no significant differences between male and female students. Data were also analyzed using multiple regression and factor analysis. The results do not corroborate basic premises of Piagetian theory and indicate the need for neo-Piagetian views to explain cognitive development.