Higher order cognitive development and success in the study of high school mathematics and science require an understanding of rational number concepts and facility with proportional reasoning and computation. Proportional reasoning is an essential schema for developing formal operational thought. This study involving 16 ninth-grade students was conducted to investigate the following questions: (1) Does knowing a standard algorithm for solving proportion problems interfere with the development of proportional reasoning? (2) Are better mathematics students more flexible or intuitive than weaker students in applying proportional reasoning strategies to solve problems? and (3) What is the relationship between a student's general level of reasoning ability (concrete, transitional, or formal) and the strategies used for solving proportional reasoning tasks? Students engaged in proportional reasoning tasks and computational proportional problem solving in their science classes made a general positive gain in Lawson's Classroom Test of Scientific Reasoning scores, although gains for concrete learners were mixed. Explicit teaching of the concept of ratios and student engagement in exploratory studies of the relationships between and among ratios provoked development of proportional reasoning for average students. Students can benefit from experiences from which the cross-multiply-and-divide algorithm can be derived. (Contains 17 references.) (Author/MDH)