Many students' knowledge of fractions is adversely affected by whole number bias, the tendency to focus on the separate whole number components (numerator and denominator) of a fraction rather than on the fraction's integrated magnitude (ratio of numerator to denominator). Although whole number bias appears early in the fraction learning process and persists into adulthood even among mathematicians, little is known about its development. The present study demonstrates that with age and experience, whole number bias decreases, and reliance on fractions' integrated magnitudes increases, on both number line estimation and magnitude comparison tasks. In particular, children treated equivalent fractions with larger components (e.g., 16/20) as larger than ones with smaller components (e.g., 4/5), thus indicating a whole number bias, but the amount of bias decreased considerably between fourth and eighth grade. Implications of the findings for children's understanding of fraction equivalence and for theories of numerical development are discussed. At the time of submission to ERIC, this article was in press with "Developmental Science."]