At issue in this study was the extent to which large numbers of classroom teachers were able to implement research-based materials with a minimum of inservice education. Also at issue was whether students of these teachers were able to develop the rich mental images for fractions similar to the ones students from previous Rational Number Project (RNP) studies developed in smaller experimental settings. An analysis of student interviews (n=20) in fourth grade in a suburban school district south of St. Paul and Minneapolis, Minnesota demonstrated that RNP students did in fact develop rich mental images for fractions similar to students in previous studies. As expected, the nature of RNP students' thinking about rational numbers was far richer and indicated a more conceptually oriented framework than that of students who used textbook curriculum. (Author/MKR)