This paper reports on a study of seventh grade students (N=4) who attend a rural K-12 school. Students participated in a 5-week teaching experiment designed to build on their existing knowledge of the unit concept and extend it to the rational number operations of multiplication and division. Data collected comes in the form of videotapes, audiotapes, researcher journal, and students' written work. Other data consist of individual student interviews conducted prior to and at the conclusion of the teaching experiment. Analysis of the results reveals that students' concepts of unit are enriched by participating in the teaching experiment. Among the conclusions drawn were that students developed a flexible concept of unit, modeling provided continuity between conceptual domains, equipartitioning remained a persistent difficulty, sustained focus on the measuring unit is hard to achieve, development of models is impeded by students' selection and use of measurement units, unitizing skills endure and are extendible, and models can inform procedural methods and/or provide alternative solution methods. (DDR)