The purpose of this causal-comparative study was to gain insight into whether teaching fractions improves teachers' understanding of fractions. University master's students (n = 25) conducted tutoring focused on fraction magnitude using number line representations across 39 sessions (3 per week) at grades 3-5. The contrast condition, 17 master's students drawn from the same pool of research assistant applicants, conducted tutoring designed to improve science and social studies text comprehension over 42 sessions (3 per week) at grades 3-5. In the fractions condition, students were at risk for mathematics difficulty; in the text comprehension condition, at-risk for reading difficulty. Across conditions, tutoring was similarly structured, with ongoing support to implement methods with fidelity while permitting flexibility to address individual student needs. When tutoring ended, accuracy of number line placement of single fractions and fraction sums as well as single whole numbers and whole number sums was assessed. Results indicated superior accuracy for fraction tutors over text comprehension tutors on fraction sums, with an effect size of 0.75. The effect size for placement of single fractions was 0.47. Implications are extrapolated from this study's sample of tutors to preservice and inservice teachers. [This paper is the in press version of an article that will be published in "The Elementary School Journal."]