The purpose of this study was to develop a paper-pencil test of Piagetian levels of proportional thinking for junior high school students in the context of physical science. Two thousand twenty-seven students were tested to develop the instrument and the description of its characteristics. The final form consisted of 24 items with four subtests each of six items for Piagetian levels: Concrete Operational I, Concrete Operational II, Formal Operational I, and Formal Operational II. Piagetian task interviews were also given to a group of students, and the paper-pencil test results correlated positively with the task results of the students who took both tests. Content, concurrent construct, divergent, and convergent validity measurements showed the paper-pencil test to be valid. The test was also shown to have a high reliability and good item discrimination between proportional reasoning levels. (MH)