This study used units coordination as a theoretical lens to investigate how whole number and fraction reasoning may be related for preservice teachers at the conclusion of a math methods class. The study contributes quantitative evidence that units coordination provides a common foundation for both mathematical knowledge for teaching whole number addition and subtraction and for mathematical knowledge for teaching fraction multiplication and division. An analysis of PSTs' written responses to two different types of tasks (referent unit and appropriateness of proportional reasoning) revealed that some aspects of mathematical knowledge for teaching rational number and the multiplicative conceptual field are associated with units coordination (referent unit tasks), whereas others are not (appropriateness tasks). A logistic regression of PSTs' conceptions of multidigit number on their scores on these tasks provided empirical evidence that the most sophisticated conceptions of multidigit number (corresponding with having interiorized three levels of units) were associated with referent unit tasks but not associated with appropriateness of proportional reasoning tasks. Together, these findings support the claim that units coordination has promise for explaining the relationship between teachers' mathematical knowledge for teaching multidigit addition and subtraction and fraction multiplication and division. Implications for teacher education are discussed.