The purpose of the present study was to reveal the developmental pathway from third grade cognitive competencies to sixth grade conceptual and procedural fraction knowledge through the intervening whole numerical skills at fifth grade. The study used empirical data that come from 536 students in nine schools across two Delaware public school districts. Students were followed longitudinally for four years, beginning in third grade and ending in sixth grade. All students in participating schools were taught with curricula aligned with the Common Core State Standards in Mathematics (Council of Chief State School Officers & National Governors Association Center for Best Practices, 2010). Mediation analysis, following a sequential structural equation modeling (SEM) approach, was used to examine the relationships among third grade general cognitive competencies, fifth grade whole number related skills, and sixth grade fraction knowledge. By using a sequential SEM design, the authors found that domain general competencies is important for developing fraction knowledge only in that they are direct precursors for intervening whole-number related skills, which in turn promote understanding of fractions. More specifically, it is through whole number magnitude understanding, rather than through whole number arithmetic, that cognitive competencies indirectly influenced sixth graders' knowledge in fraction concepts; attentive behavior and verbal ability at early stages are important for developing whole number arithmetic, which, in turn, are relevant for learning fraction procedures. A table and figures are appended. [SREE documents are structured abstracts of SREE conference symposium, panel, and paper or poster submissions.]