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 on fraction magnitude in grades 3-5. The contrast condition, 17 master's students drawn from the same pool of research assistant applicants, conducted tutoring to improve science and social studies text comprehension in grades 3-5. In the fractions condition, students were at risk for mathematics difficulty; in the text comprehension condition, at risk for reading difficulty. When tutoring ended, tutors' 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 to preservice and in-service teachers.