The purpose of this study was to investigate how prospective elementary school mathematics teachers and elementary school students' ability to distinguish proportional situations from nonproportional situations differ or are alike. Participants of the study were 319 prospective primary and middle school teachers and 320 elementary school students from fourth to seventh grades. Four problems consisting of two proportional and two nonproportional situations were used as a data collection tool. Participants' strategies were coded as multiplicative, additive, and other. Findings indicated that while fourth, fifth, and sixth graders were dominantly additive reasoners, prospective teachers and seventh graders were multiplicative reasoners. In addition, it was found that the overgeneralization of proportionality seems to begin with the instruction on the cross-multiplication algorithm. Implications for instruction on proportion, mathematics teacher education programs and future research are discussed.