The current study examines the extent to which the whole number bias, especially whole number ordering, can interfere with adult understandings of fractions. Using the framework theory approach to conceptual change as outlined by Vosniadou (2007; Vosniadou, Vamvakoussi & Skopeliti, 2008), this study supports the idea that initial concepts formed in childhood can have lasting effects into adulthood. Twenty-eight CMU undergraduates participated in a fraction magnitude comparison task. Half of the fraction comparisons were designed with the larger fraction consistent with whole number ordering; the other half was inconsistent with this ordering. Comparisons in the consistent condition had the larger magnitude fraction have larger whole number parts than the opposing fraction. Comparisons in the inconsistent condition were the opposite. Participants were more accurate and faster to respond to comparisons in the consistent condition, supporting the hypothesis that an initial concept of number as natural number constrains operations with fractions even in adults.