We report on results of a classroom teaching experiment in a mathematics content course for prospective elementary teachers. A local instruction theory for the development of number sense, which was previously applied to whole-number mental computation, was extended to inform instruction concerning reasoning about fraction magnitude. We found that students' reasoning in fraction comparison tasks improved in several ways. Their performance improved, they became more flexible in their reasoning, and they came to use less conventional and more sophisticated strategies. These changes parallel those that we previously saw around mental computation. [For the complete proceedings, see ED585874.]