This study had two research purposes. First, we examined the scientific reasoning gains of prospective science teachers who are concrete, formal, and postformal reasoners in an argumentation-based physics inquiry instruction. Second, we sought conceptual knowledge and achievement gaps between these student groups before and after the instruction. Results were reported for 114 prospective science teachers. Results showed that concrete reasoners' scientific reasoning gain was higher than those of formal and postformal reasoners. Moreover postformal reasoners outperformed formal and concrete reasoners on a situational conceptual knowledge subscale before and after instruction. In addition, postformal and formal reasoners scored higher than concrete reasoners both on an initial achievement and final achievement measures. However, in-depth analyses showed that final achievement differences between postformal and concrete, and formal and concrete reasoners were lower than their respective initial achievement differences. Implications for teacher education programs were discussed according to these findings.